libadmin: use INSTALL_DATA to export headers

Use the INSTALL_DATA macro instead of cp to export
header files.

Reviewed-on: http://gerrit.openafs.org/10515
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
(cherry picked from commit 66093e4a2db297afea755d164cca7a6080909bda)

Change-Id: Icc1fb62fd68903fcf5a1f756800b0281669f59fa
Reviewed-on: http://gerrit.openafs.org/11234
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>