Qt: Disable controller configuration while NetPlay is running

Doing pretty much anything in the controller config breaks NetPlay
(desync and/or deadlock), as saving the settings reconfigures
controller interfaces, which NetPlay doesn't expect.

Source/Core/DolphinQt/MenuBar.h

@@ -215,6 +215,7 @@ private:
   QAction* m_boot_to_pause;
   QAction* m_automatic_start;
   QAction* m_change_font;
+  QAction* m_controllers_action;
 
   // View
   QAction* m_show_code;