Fix state updates for tabs

This broke when I introduced the scrolling. We need to
iterate over the children of the tabs box to update their
state.

gtk/gtktabstrip.c

@@ -471,7 +471,7 @@ gtk_tab_strip_stack_notify_visible_child (GtkTabStrip *self,
   visible_child = gtk_stack_get_visible_child (stack);
 
   priv->in_child_changed = TRUE;
-  gtk_container_foreach (GTK_CONTAINER (self), update_visible_child, visible_child);
+  gtk_container_foreach (GTK_CONTAINER (priv->tabs), update_visible_child, visible_child);
   priv->in_child_changed = FALSE;
 }