places sidebar: Always chain up in finalize

The GtkSidebarRow class was forgetting to do this. The visible
symptom of this in valgrind is that you notice GObject data
being leaked.

gtk/gtksidebarrow.c

@@ -298,6 +298,8 @@ gtk_sidebar_row_finalize (GObject *object)
   g_clear_object (&self->drive);
   g_clear_object (&self->volume);
   g_clear_object (&self->mount);
+
+  G_OBJECT_CLASS (gtk_sidebar_row_parent_class)->finalize (object);
 }
 
 static void