rfc3961: prototype _krb5_internal_hmac

rfc3961: prototype _krb5_internal_hmac

Resolves this warning:

src/external/heimdal/krb5/crypto-arcfour.c: In function ‘_oafs_h__krb5_HMAC_MD5_checksum’:
src/external/heimdal/krb5/crypto-arcfour.c:82:5: warning: implicit declaration of function ‘_oafs_h__krb5_internal_hmac’ [-Wimplicit-function-declaration]
     ret = _krb5_internal_hmac(context, c, signature, sizeof(signature),
     ^

Change-Id: I10f028b8a0e1756cb1f1638a061616db0e76779e
Reviewed-on: http://gerrit.openafs.org/11953
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>