monitor-manager: Shut up a compiler warning

Add a default switch statement to make it clear that 'transform'
won't be used uninitialized.

src/backends/meta-monitor-manager.c

@@ -654,6 +654,7 @@ orientation_changed (MetaOrientationManager *orientation_manager,
       break;
 
     case META_ORIENTATION_UNDEFINED:
+    default:
       return;
     }