Install afscp.h from srcdir, not the build dir

Install afscp.h from srcdir, not the build dir

That header is not a generated file, and is not found in the
object directory.  make install would fail from a separate objdir
prior to this change.

Change-Id: I31041c793bd930cfe2fc3c5f8a754ba1b91c26e5
Reviewed-on: http://gerrit.openafs.org/11160
Reviewed-by: D Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>