sentinel-nikola: fix messages filter in syslog-ng

Without this change iptables messages are included in /var/log/messages as well as in our target. That means without this we are not filtering them from default log. The reason is that source for standard log is defined before source for nikola's log. This means that filter is applied late.

Syslog-ng's scls are included before main config while standard configs are appended. Including them so soon makes filter effective.