Add missing prototype for _gcry_mac_selftest

* src/cipher-proto.h (_gcry_hmac_selftest): Rename to...
(_gcry_mac_selftest): ... this.

It appears that '_gcry_hmac_selftest' has been renamed to
'_gcry_mac_selftest' but renaming prototype was missed.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>