aklog: can't assume krb5_524_conv_principal based on convert_creds

aklog: can't assume krb5_524_conv_principal based on convert_creds

add probing for krb5_524_conv_principal directly instead of
assuming finding a 524 library will find it. current heimdal
does not include it.

Reviewed-on: http://gerrit.openafs.org/7551
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Derrick Brashear <shadow@dementix.org>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit 6d3390bc16281e6f889bb555abb1182985556287)

Change-Id: Icc576d1a711fa771fda393c61a15082ffb94defc
Reviewed-on: http://gerrit.openafs.org/7582
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>