tbudb depends on libafsauthent

budb_server in src/tbudb requires libafsauthent to build. So, make
tbudb depend on libafsauthent.

This is a 1.6-only change. On master, tbudb no longer uses
libafsauthent.

Change-Id: I0737f455323589c28a78d680c6c09fcbcb99d302
Reviewed-on: http://gerrit.openafs.org/9456
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>