mosquitto: Include viable stub config file

This should always have been available in the mosquitto package itself.
This file needs to exist in basic form to allow the init scripts to even check whether it exists.

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

net/mosquitto/files/etc/config/mosquitto

@@ -0,0 +1,4 @@
+config owrt owrt
+    option use_uci 0
+
+config mosquitto mosquitto