macosx: correctly dis/enable cover art menu item

(cherry picked from commit 319610b3107723376ecb41d6c7562059590b81a7)
Signed-off-by: David Fuhrmann <dfuhrmann@videolan.org>

modules/gui/macosx/playlist.m

@@ -1325,6 +1325,7 @@
     [o_mi_recursive_expand setEnabled: b_item_sel];
     [o_mi_sort_name setEnabled: b_item_sel];
     [o_mi_sort_author setEnabled: b_item_sel];
+    [o_mi_dl_cover_art setEnabled: b_item_sel];
 
     return o_ctx_menu;
 }