efl_ui_group_item: fix insertion of item at the end

efl_ui_group_item: fix insertion of item at the end

Summary:
in this case here, eina list count already returns the count *including*
the subobject, the case in pack_at below (where also +1 is added) is
different to that, as count is the value *before* the list add.
Depends on D9870

Reviewers: segfaultxavi, zmike, cedric

Reviewed By: zmike

Subscribers: #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D9871