ssh-agent: Fix leak in op_v1_request_identities

The object returned by gck_enumerator_next() must be unref'ed
as this method is "transfer full".

https://bugzilla.gnome.org/show_bug.cgi?id=738508
ssh-agent: Fix leak in op_request_identities

The object returned by gck_enumerator_next() must be unref'ed.

https://bugzilla.gnome.org/show_bug.cgi?id=738508