Fixes compilation with the forthcoming libgcrypt 1.6.x.
Reported by Matthias Wachs <wachs@net.in.tum.de>
and NIIBE Yutaka <gniibe@fsij.org>.
* nix/libutil/gcrypt-hash.cc (guix_hash_init, guix_hash_final): Use
'int' as the type of the 'algo' parameter.
* nix/libutil/gcrypt-hash.hh: Update declarations accordingly.
* nix/libutil/md5.h (MD5_Final): Pass RESBUF as the first argument to
`guix_hash_final'.
* nix/libutil/sha1.h (SHA1_Final): Likewise.
* nix/libutil/sha256.h (SHA256_Final): Likewise.