rra-c-util: Add additional file

rra-c-util: Add additional file

Add the m4/krb5-config.m4 file, which is required for version 4.4,
to the list of imports.

Change-Id: Ib5d3d0d7be67eaa75f6edc6d030cdb3c47838d3c
Reviewed-on: http://gerrit.openafs.org/7400
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Russ Allbery <rra@stanford.edu>
Reviewed-by: Derrick Brashear <shadow@dementix.org>