Check for keyblock init functions in libkrb5

MIT provides krb5_init_keyblock, heimdal krb5_keyblock_init.
We'll need to create a keyblock from an external key in order to
use the RFC 3961 crypto routines from krb5 for rxgk.

Change-Id: Ia8453d33fb71ee7502cb4ee4f20cfccf00ed1195