fosquitto: fix file watch without parameter

This variable is not defined and for sure it was suppose to be /etc/config/fosquitto.

In previous versions of procd it is probably silently ignored this invalid request but in new version it siletly ignores whole instance start. This means that this bug caused fosquitto to not start. Fixes https://gitlab.labs.nic.cz/turris/turris-build/issues/103