Suppress a warning for g_object_ref_sink() use in widgets sample

The glib macro always triggers this warning when -Wextra is on, so we
have no choice but to suppress it.

samples/widgets/native.cpp

@@ -170,7 +170,9 @@ public:
         );
 #endif // GTK+ 3.6/earlier
 
+        wxGCC_WARNING_SUPPRESS(ignored-qualifiers)
         g_object_ref_sink(widget);
+        wxGCC_WARNING_RESTORE(ignored-qualifiers)
 
         (void)Create(parent, wxID_ANY, widget);
     }