package/mosquitto: security bump to v1.6.12

package/mosquitto: security bump to v1.6.12

Mosquitto 1.6.11 is a bugfix release, read the whole announcement on
http://mosquitto.org/blog/2020/08/version-1-6-11-released/

Mosquitto 1.6.12 is a security and bugfix release, read
http://mosquitto.org/blog/2020/08/version-1-6-12-released/

>From the 1.6.11 changelog of the client library:
mosquitto_loop_start() now sets a thread name on Linux, FreeBSD, NetBSD,
and OpenBSD. Closes #1777.
This is done with pthread_setname_np; so mosquitto now requires
BR2_TOOLCHAIN_HAS_THREADS_NPTL when built with threading support.

2 reverse dependencies use the threaded API, but they already
depend on BR2_TOOLCHAIN_HAS_THREADS_NPTL:
* domoticz [1] (we add a comment for mosquitto)
* shairport-sync [2]

[1] https://github.com/domoticz/domoticz/blob/2020.1/main/mosquitto_helper.cpp#L344
[2] https://github.com/mikebrady/shairport-sync/blob/3.3.6/mqtt.c#L227-L229

Signed-off-by: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>