cipher: Enable the fast path to ChaCha20 only when supported.
cipher/cipher-poly1305.c (_gcry_cipher_poly1305_encrypt)
(_gcry_cipher_poly1305_decrypt) [USE_CHACHA20]: Conditionalize.
- GnuPG-bug-id: T6384
- Reported-by: Andrew Collier
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>