Silence compiler warnings for possible alignment problem.

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

In the first case, we know that allocated area by calloc is properly
aligned. We use unsigned char * for LAST_BLOCK for address
computation.

In the latter cases, 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>