syslog-ng: add source net section for the upcoming feature

By term upcoming feature, I mean, we need this section in Turris OS 5.x, but there is already present, but it is missing in Turris OS 3.x. It results if you are using Storage plugin then you can redirect the syslog to /srv, but if there is missing source net, syslog-ng does not start.

This should cover migration issue, however, syslog-ng.conf is conffile which things complicated.

Fixes: https://forum.turris.cz/t/turris-os-5-2-0-is-in-hbt-testing-branch/15081/25?u=pepe