systemd: Add a Restart=on-failure option to the service file

In case syslog-ng goes down due to an unexpected signal, or perhaps a
crash, we want logging to resume, so tell systemd to restart the
service on failure.

Reported-by: Paul Dann <pdgiddie+balabit@gmail.com>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>