aklog: Fix error message fallback for Heimdal

aklog: Fix error message fallback for Heimdal

Since we reverted to using Russ's upstream version of rra-c-util's
krb5.m4, we no longer check for the existence of the pure com_err
error_message().

So, for error message fallback on Heimdal, use krb5_get_error_message()
instead. As we don't have access to the context in which the error
occurred, build one to get the raw com_err error translation - this
won't give us any additional context specific detail.

Change-Id: I4bab080704ec5d08e8b85e6b7ad69cf1f89bf3b3
Reviewed-on: http://gerrit.openafs.org/7554
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>