gpg: For decryption, support use of a key with no 'encrypt' usage.
* g10/pubkey-enc.c (get_session_key): Don't skip at no PUBKEY_USAGE_ENC. Emit information the key has no 'encrypt' usage.
- GnuPG-bug-id: T4246
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>