nixos/shairport-sync: restart the systemd service on failure

This ensures shairport-sync is running, even after crashes.

nixos/modules/services/networking/shairport-sync.nix

@@ -103,6 +103,7 @@ in
           User = cfg.user;
           Group = cfg.group;
           ExecStart = "${lib.getExe cfg.package} ${cfg.arguments}";
+          Restart = "on-failure";
           RuntimeDirectory = "shairport-sync";
         };
       };