gpg: Support exporting Ed448 SSH key.
* common/openpgp-oid.c (oid_ed448, openpgp_oidbuf_is_ed448): New. (openpgp_oid_is_ed448): New. * common/util.h (openpgp_oid_is_ed448): New. * g10/export.c (export_one_ssh_key): Support Ed448 key.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>