desktop-shell: Don't call wl_list_init() in the middle of a list

seat->keyboard_focus_listener.link isn't a head, it's just sometimes a
member of the focus signal list.  Calling wl_list_init() on it puts
a loop in the list.

Instead, we remove the item then init it.  That way we can call remove on
it again later even if it hasn't been re-added to a list.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>