3.5/f/cfg tree final fix

This should fix a config compilation problem, and affects both 3.4 and 3.5, and probably be fixed in both.

@Algernon: could you please apply this to both 3.4 and 3.5? The bug was reported on the mailing list two weeks ago with the subject: [syslog-ng] syslog-ng 3.5.1 - question about flags(final)...

I'm integrating the same fix to 3.6/master. Thanks.