scd: Use the correct gpg for the v1.0 OpenPGP card hack.

* scd/app-openpgp.c (get_public_key): Use gnupg_module_name instead of
just "gpg".

There is no bug report regarding this and it would be very unlikely
but we should always use the gpg belonging to our code.

• Signed-off-by: Werner Koch <wk@gnupg.org>