Rename hmac() function

NetBSD 8's stdlib.h declares a hmac() function; rename ours to avoid a
conflict.

[ghudson@mit.edu: picked a different name and added a comment]

https://github.com/krb5/krb5/commit/e8b463c8d2c9b903d61c8646af3c0106808f906d
Author: Thomas Klausner <tk@giga.or.at>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: e8b463c8d2c9b903d61c8646af3c0106808f906d
Branch: master
 src/lib/crypto/builtin/pbkdf2.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)