Win32: add extra parenthesis for gcc-win32 4.4.1

I thought the C priorities were clear on that kind of figure... Why through an error then?
But, then, why not? It doesn't clutter the code that much and improves fast readability...
Maybe Diego is right... I don't know.

modules/video_output/msw/events.c

@@ -1361,6 +1361,6 @@ void StopEventThread( vout_thread_t *p_vout )
         vlc_object_release( p_vout->p_sys->p_event );
     }
 
-    if( !p_vout->p_sys->i_changes & SWITCHING_MODE_FLAG )
+    if( !( p_vout->p_sys->i_changes & SWITCHING_MODE_FLAG ) )
         vlc_mutex_destroy( &p_vout->p_sys->lock );
 }