feeds-pool: fix introspection for feed-ready signal

last parameter for 'feed-ready' signal is GList of GrssFeedItem, but
introspection shows us that it is GList of gpointer. Let's fix this.

Tested-by: Vadim Rutkovsky <vrutkovs@redhat.com>
Signed-off-by: Igor Gnatenko <ignatenko@src.gnome.org>