aklog: Probe for libasn1 on heimdal

aklog uses encode_EncTicketPart and some other encode_* ASN.1 routines
when we're building against heimdal. Our krb5 autoconf logic from
c-rra-util is not guaranteed to include libasn1 in KRB5_LIBS, since
it's not required for functions in the krb5 API. So, specifically test
for it.

Related issue reported by Måns Nilsson.

Change-Id: I5d1ab07ec481e48710bafcdc53fe58f529cc6dde
Reviewed-on: http://gerrit.openafs.org/9693
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>