No need to explicitely include relation.lib its anyway included in common.inc

main.php

@@ -309,7 +309,6 @@ if (file_exists('./config')) {
  * Check whether relations are supported.
  */
 if ($server > 0) {
-    require_once './libraries/relation.lib.php';
     $cfgRelation = PMA_getRelationsParam();
     if(!$cfgRelation['allworks'] && $cfg['PmaNoRelation_DisableWarning'] == false) {
         $message = PMA_Message::notice(__('The additional features for working with linked tables have been deactivated. To find out why click %shere%s.'));