Use rk_mkdir from libafsroken

Use rk_mkdir from libafsroken

The latest import added it; add it to the build and export list.

Change-Id: I588de000868d063c61753a1a4a2e73931e84005f
Reviewed-on: http://gerrit.openafs.org/10046
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>