For now, disable mqtt as it was automatically enabled as the build
system finds compiled libpaho-mqtt-c and requires dependency.

---
Here is the output:
Package syslog-ng is missing dependencies for the following libraries:
libpaho-mqtt3c.so.1
---

This is a new feature since syslog-ng 3.33.1 and if anyone is interested
in it, it can be enabled.

Signed-off-by: Josef Schlehofer <pepe.schlehofer@gmail.com>