net/mosquitto: bump to 1.6.7

net/mosquitto: bump to 1.6.7

Fixes compatibility with newer libwebsockets versions.
Fixes performance issues with websockets
Fixes potential crash on config reload
Fixes CVEs: CVE-2019-11778 and CVE-2019-11779
Various MQTTv5 fixes.
Skips releases with critical regressions

Changelog for 1.6.7:
https://mosquitto.org/blog/2019/09/version-1-6-7-released/

Changelog for 1.6.5:
https://mosquitto.org/blog/2019/09/version-1-6-5-released/

Changelog for 1.6.6:
https://mosquitto.org/blog/2019/09/version-1-6-6-released/

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