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]

(cherry picked from commit e8b463c8d2c9b903d61c8646af3c0106808f906d)

https://github.com/krb5/krb5/commit/bfc4bcaf6b132e12d15b0855c1cc71ddbe52c554
Author: Thomas Klausner <tk@giga.or.at>
Committer: Greg Hudson <ghudson@mit.edu>
Commit: bfc4bcaf6b132e12d15b0855c1cc71ddbe52c554
Branch: krb5-1.17
 src/lib/crypto/builtin/pbkdf2.c |   10 ++++++----
 1 files changed, 6 insertions(+), 4 deletions(-)