nixos/mautrix-facebook: create group

Otherwise we get a warning when applying the config.

nixos/modules/services/matrix/mautrix-facebook.nix

@@ -116,6 +116,8 @@ in {
   };
 
   config = mkIf cfg.enable {
+    users.groups.mautrix-facebook = {};
+
     users.users.mautrix-facebook = {
       group = "mautrix-facebook";
       isSystemUser = true;