gpg: Ed448 and X448 are only for v5 for --full-gen-key.

* g10/keygen.c (generate_keypair): Set pVERSION = 5, pSUBVERSION = 5,
when it's Ed448 or X448.

• Fixes-commit: 36355394d865f5760075e62267d70f7a7d5dd671
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>