Suppress a GCC -Wcast-function-type warning

We're setting the same event handler GTK uses internally, so the warning isn't helpful

src/gtk/evtloop.cpp

@@ -380,7 +380,10 @@ void wxGUIEventLoop::DoYieldFor(long eventsToProcess)
     gdk_event_handler_set(wxgtk_main_do_event, this, NULL);
     while (Pending())   // avoid false positives from our idle source
         gtk_main_iteration();
+
+    wxGCC_WARNING_SUPPRESS(cast-function-type)
     gdk_event_handler_set ((GdkEventFunc)gtk_main_do_event, NULL, NULL);
+    wxGCC_WARNING_RESTORE()
 
     wxEventLoopBase::DoYieldFor(eventsToProcess);