libadmin: makefile rule for afs_AdminError.h

Add a makefile rule to export the libadmin afs_AdminErrors.h header
file, instead of exporting afs_AdminErrors.h as a side effect of
generating the afs_AdminBosErrors error table.

Add the missing afs_AdminErrors.h dependency to the afs_utilAdmin.o
dependency list.

Reviewed-on: http://gerrit.openafs.org/10369
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 9b1ffeeeb62cd9535dc2fc1522400abb428a0ac2)

Change-Id: Ib526d18227a6c37eac08def5ab88eb1717a848f1
Reviewed-on: http://gerrit.openafs.org/11233
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>