database: make parent tables work with plugin

Enables the use of PictureFlow and the Properties plugin
with parent tables of ALLSUBENTRIES, such as an album
or album artist, instead of individual tracks.

Change-Id: I18c4779ed116a48c732ae32b9629e7e0d93ce7c8

apps/plugin.c

@@ -825,6 +825,9 @@ static const struct plugin_api rockbox_api = {
     splash_progress_set_delay,
     fix_path_part,
     onplay_show_playlist_cat_menu,
+#if defined(HAVE_TAGCACHE)
+    tagtree_subentries_do_action,
+#endif
 };
 
 static int plugin_buffer_handle;