package/mosquitto: bump to v1.6.9

mosquitto 1.6.9 is a bugfix release, see the announcement:
https://mosquitto.org/blog/2020/02/version-1-6-9-released/



Also update the indentation of the hash file to 2 spaces,
and add URL of the GPG signature in hash file comment.

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