system-source: Do not use /dev/kmsg when using systemd

When using the Journal, do not use /dev/kmsg, because that is included
in the journal too, and using it would lead to log duplication.

Reported-by: Andras Mitzki <micek@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>