GtkSearchEngine: Avoid a crash

Add a destroy notify for the data of the callback, so we don't
end up leaving a dangling pointer behind for a short while if
the native engine is finalized before the simple one. This
was showing up as crash when typing and backspacing in the
search entry of the file chooser.

gtk/gtksearchenginesimple.h

@@ -57,7 +57,8 @@ typedef gboolean (*GtkSearchEngineSimpleIsIndexed) (GFile *location, gpointer da
 
 void             _gtk_search_engine_simple_set_indexed_cb (GtkSearchEngineSimple *engine,
                                                            GtkSearchEngineSimpleIsIndexed callback,
-                                                           gpointer                       data);
+                                                           gpointer                       data,
+                                                           GDestroyNotify                 destroy);
 
 G_END_DECLS