nixos/synergy: Restart services on failure.

Synergy seems to get more and more unstable in recent versions, so we
might want to debug this properly. However, it makes sense to restart
the service nevertheless, because synergy is about keyboard and mouse
sharing and it's quite annoying to either SSH in to restart the service
or even needing to unplug the keyboard and plug in into the machine with
the failing service.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>