net/mosquitto: update to 1.5.1

net/mosquitto: update to 1.5.1

Bugfix release.  Full changelog at:
https://mosquitto.org/blog/2018/08/version-151-released/

Of most interest to OpenWrt:
* Remove use of AI_ADDRCONFIG, which means the broker can be used on systems where only the loopback interface is defined.
* Fix IPv6 addresses not being able to be used as bridge addresses.
* Fix problem opening listeners on Pi caused by unsigned char being default.
* Fix segfault on startup if bridge CA certificates could not be read.
* Fix possible endian issue when reading the memory_limit option.
* library and client bugfixes including: https://github.com/openwrt/packages/issues/6765

Signed-off-by: Karl Palsson <karlp@etactica.com>