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.

Change-Id: Iea523e3e7ba4cb51ec5a0d4b38fec9c7df30529d
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>