sqlite3: fix uClibc builds

sqlite3: fix uClibc builds

When compiling against uClibc on lede-17.01 it's detected in the linking
phase that '__isnan' is nowhere to be found:

sqlite3-sqlite3.o: In function `serialGet':
sqlite3.c:(.text+0x6364): undefined reference to `__isnan'
sqlite3-sqlite3.o: In function `sqlite3_result_double':
sqlite3.c:(.text+0x10faa): undefined reference to `__isnan'
sqlite3-sqlite3.o: In function `sqlite3VXPrintf':
sqlite3.c:(.text+0x175ca): undefined reference to `__isnan'
sqlite3-sqlite3.o: In function `sqlite3_bind_double':
sqlite3.c:(.text+0x1b0ac): undefined reference to `__isnan'
sqlite3-sqlite3.o: In function `sqlite3VdbeExec':
sqlite3.c:(.text+0x3b77e): undefined reference to `__isnan'
collect2: error: ld returned 1 exit status

To fix this libm needs to be linked in as well in the uClibc case. So
add libm ('-lm') to the TARGET_LDFLAGS accordingly.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>