-Add missing yield use statement to copycds plugin

git-svn-id: https://svn.code.sf.net/p/xcat/code/xcat-core/trunk@1953 8638fb3e-16cb-4fca-ae20-7b5d299a9bcd

xCAT-server/lib/xcat/plugins/copycds.pm

@@ -4,6 +4,7 @@ use strict;
 use warnings;
 use Storable qw(dclone);
 use xCAT::Table;
+use Thread qw(yield);
 use Data::Dumper;
 use Getopt::Long;
 use File::Basename;