mosquitto: bump to 2.0.15

Changelog: https://mosquitto.org/blog/2022/08/version-2-0-15-released/
Changelog: https://mosquitto.org/blog/2021/11/version-2-0-14-released/

2.0.15 is bigger security and bugfix release.  2.0.14 had a couple of
  minor changes and was skipped for OpenWrt.

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