keccak: add hash_buffers functions for SHAKE128 & SHAKE256
* cipher/keccak.c (_gcry_shake128_hash_buffers) (_gcry_shake256_hash_buffers): New. (_gcry_digest_spec_shake128, _gcry_digest_spec_shake256): Add hash_buffers pointer.
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>