services.redshift: Enable auto-restart.

The redshift service stops working after some time (the program exits
after some hours/days). It looks like these exits are related to calls
to xrandr (for multiple displays) or suspend-to-ram.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>

nixos/modules/services/x11/redshift.nix

@@ -46,6 +46,7 @@ in {
           -t ${toString cfg.temperature.day}:${toString cfg.temperature.night}
       '';
       environment = { DISPLAY = ":0"; };
+      serviceConfig.Restart = "always";
     };
   };
 }