Bug 16505: Make sure $as_xml will not be used later

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>

misc/migration_tools/rebuild_zebra.pl

@@ -100,6 +100,7 @@ if (not $result or $want_help) {
 
 if ( $as_xml ) {
     warn "Warning: You passed -x which is already the default and is now deprecated·\n";
+    undef $as_xml; # Should not be used later
 }
 
 if( not defined $run_as_root and $run_user eq 'root') {