net: seafile-server: add libevent2 deps on PKG_BUILD_DEPENDS

net: seafile-server: add libevent2 deps on PKG_BUILD_DEPENDS

And libevent2-pthreads to DEPENDS.

While building with -j12 I seem to get:
[ sometimes this also includes libevent2-openssl ]

```
-- EVHTP_VERSION            :  1.2.18
-- EVHTP_DISABLE_SSL        :  OFF
-- EVHTP_DISABLE_EVTHR      :  OFF
-- EVHTP_DISABLE_REGEX      :  OFF
-- EVHTP_BUILD_SHARED       :
-- EVHTP_USE_JEMALLOC       :
-- EVHTP_USE_TCMALLOC       :

-- CMAKE_BUILD_TYPE         : Release
-- CMAKE_INSTALL_PREFIX     : /usr
-- CMAKE_BINARY_DIR         : /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18
-- CMAKE_CURRENT_BINARY_DIR : /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18
-- CMAKE_CURRENT_SOURCE_DIR : /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18
-- PROJECT_BINARY_DIR       : /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18
-- PROJECT_SOURCE_DIR       : /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18
-- CMAKE_MODULE_PATH        : /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/cmake
-- CMAKE_SYSTEM_NAME        : Linux
-- CMAKE_SYSTEM_VERSION     : 1
-- CMAKE_C_COMPILER         : /home/sandu/work/openwrt/openwrt/staging_dir/toolchain-i386_pentium4_gcc-8.4.0_musl/bin/i486-openwrt-linux-musl-gcc
-- CMAKE_AR                 : /home/sandu/work/openwrt/openwrt/staging_dir/toolchain-i386_pentium4_gcc-8.4.0_musl/bin/i486-openwrt-linux-musl-gcc-ar
-- CMAKE_RANLIB             : /home/sandu/work/openwrt/openwrt/staging_dir/toolchain-i386_pentium4_gcc-8.4.0_musl/bin/i486-openwrt-linux-musl-gcc-ranlib
-- CFLAGS                   :   -Os -pipe -march=pentium4 -fno-caller-saves -fno-plt -fhonour-copts -Wno-error=unused-but-set-variable -Wno-error=unused-result -ffile-prefix-map=/home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18=libevhtp-1.2.18 -Wformat -Werror=format-security -fstack-protector -D_FORTIFY_SOURCE=1 -Wl,-z,now -Wl,-z,relro
                              -DNDEBUG

-- Configuring done
CMake Error: The following variables are used in this project, but they are set to NOTFOUND.
Please set them or make sure they are set and tested correctly in the CMake files:
LIBEVENT_THREAD
    linked by target "evhtp" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18
    linked by target "test_proxy" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "example_basic" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "test_client" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "example_https_client" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "test_vhost" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "test_basic" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "test_extensive" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "example_chunked" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "test_query" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "test_perf" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "example_pause" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "example_https_server" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "example_vhost" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples
    linked by target "example_request_fini" in directory /home/sandu/work/openwrt/openwrt/build_dir/target-i386_pentium4_musl/libevhtp-1.2.18/examples

-- Generating done
CMake Warning:
  Manually-specified variables were not used by the project:

    CMAKE_ASM_COMPILER
    CMAKE_ASM_COMPILER_ARG1
    CMAKE_MODULE_LINKER_FLAGS
    CMAKE_SHARED_LINKER_FLAGS
    DL_LIBRARY

CMake Generate step failed.  Build files cannot be regenerated correctly.
```

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>