roundcube service: Restart on config changes.

Until now, e.g. `extraConfig` changes did not reflect in
the system on `nixos-rebuild switch`.

nixos/modules/services/mail/roundcube.nix

@@ -204,6 +204,11 @@ in
     };
     systemd.services.phpfpm-roundcube.after = [ "roundcube-setup.service" ];
 
+    # Restart on config changes.
+    systemd.services.phpfpm-roundcube.restartTriggers = [
+      config.environment.etc."roundcube/config.inc.php".source
+    ];
+
     systemd.services.roundcube-setup = mkMerge [
       (mkIf (cfg.database.host == "localhost") {
         requires = [ "postgresql.service" ];