mediawiki/syslog: qualify output channel filter

The :omfile: prefix is apparently required, based on
<http://www.rsyslog.com/doc/rsyslog_conf_output.html>.
We never had it, AFAIK, so I'm not sure why it would work before but break now,
but I figure it's worth being consistent with the docs regardless.

Change-Id: I432efc735afa4fe7767fb0e4ba061e66a2414052