libadmin: makefile rule for afs_AdminError.h

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.

Change-Id: Ib8c7d22d705061615fb20a6a521dc20f0f1d6da0
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>