main.js: Fix enum in _stageEventHandler.

This is needed as a result of an introspection fix in muffin:
linuxmint/muffin@7453e8de7689934

Author: mtwebster

js/ui/main.js

@@ -1191,7 +1191,7 @@ function _stageEventHandler(actor, event) {
     let modifierState = Cinnamon.get_event_state(event);
 
     let action = global.display.get_keybinding_action(keyCode, modifierState);
-    if (!(event.get_source() instanceof Clutter.Text && (event.get_flags() & Clutter.EventFlags.FLAG_INPUT_METHOD))) {
+    if (!(event.get_source() instanceof Clutter.Text && (event.get_flags() & Clutter.EventFlags.INPUT_METHOD))) {
         // This relies on the fact that Clutter.ModifierType is the same as Gdk.ModifierType
         if (action > 0) {
             keybindingManager.invoke_keybinding_action_by_id(action);