NetIdMgr: Krb4 is optional

NetIdMgr: Krb4 is optional

32-bit builds should not require the krb4 library.

Reviewed-on: http://gerrit.openafs.org/8414
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
(cherry picked from commit 51021ac306c7b3b355c64c978a029537e3341a25)

Change-Id: I8664140cd8e5d159789be94628a726e8c7ee5cb7
Reviewed-on: http://gerrit.openafs.org/8727
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>