meta/folder: fix options being hidden in GUI

not having a cat+subcat specified means that the options were ignored
when constructing the preferences tree in Qt (and possibly others).

(cherry picked from commit bc6700f7d1)

modules/meta_engine/folder.c

@@ -69,6 +69,8 @@ static int FindMeta( vlc_object_t * );
 vlc_module_begin ()
     set_shortname( N_( "Folder" ) )
     set_description( N_("Folder meta data") )
+    set_category( CAT_PLAYLIST )
+    set_subcategory( SUBCAT_PLAYLIST_GENERAL )
     add_loadfile( "album-art-filename", NULL,
         N_("Album art filename"), N_("Filename to look for album art in current directory"), false );
     set_capability( "art finder", 90 )