roken: Use srcdir for roken-post.h

roken: Use srcdir for roken-post.h

roken-post.h is a source file, not a generated file in the objdir.
Specify $(srcdir) so we can work with objdir builds.

Change-Id: I1d00ba1f28bea99770c2af56890fbf22ee764820
Reviewed-on: https://gerrit.openafs.org/13387
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>