hmac: Use xfree.
* src/hmac256.c [STANDALONE] (xfree): Define. (_gcry_hmac256_new, _gcry_hmac256_release): Use xfree. (_gcry_hmac256_file): Likewise.
- Fixes-commit: 3e3b520fb32a37c5c23762531a7b3168e112ac36
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>