Add missing module include.

tbsetup/checkports.in

 #!/usr/bin/perl -w
 #
 # EMULAB-COPYRIGHT
-# Copyright (c) 2000-2007 University of Utah and the Flux Group.
+# Copyright (c) 2000-2010 University of Utah and the Flux Group.
 # All rights reserved.
 #
 use English;
@@ -46,6 +46,7 @@ use lib "@prefix@/lib";
 use libdb;
 use libtestbed;
 use Experiment;
+use User;
 use snmpit_lib;
 
 # The set of OIDs we care about