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.

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>
(cherry picked from commit 7a701239d8327ae738124edb5e11f3bcbb08a76b)

Change-Id: I738adc464aa36b68403b69912253faf17cb01621
Reviewed-on: http://gerrit.openafs.org/11342
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>