Silence compiler warnings for possible alignment problem.

* cipher/kdf.c (_gcry_kdf_compute, _gcry_kdf_final, _gcry_kdf_close):
Fix the cast.

Backport master commit of:
5420cbbd3ec7ebf081224796e8d1f8299f7ad985

The object referenced by H is actually the target object of the
particular struct, so, no problem for alignment.

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>