gpg: Enable the Issuer Fingerprint from rfc4880bis

* g10/build-packet.c (build_sig_subpkt_from_sig): Always write the new
Issuer Fingerprint sub-packet.
* g10/mainproc.c (check_sig_and_print): Always consider that
sub-packet.

The specs for this sub-packet have been pushed to the OpenPGP WG's
repo today.

openpgp/GvPo2eSL9GW9WcGhOocY7KBa9FY

• See-also: https://mailarchive.ietf.org/arch/msg/\
• Signed-off-by: Werner Koch <wk@gnupg.org>