Fix error on PHP 5.3 as reported in UI tests:...

Fix error on PHP 5.3 as reported in UI tests: https://travis-ci.org/piwik/piwik/jobs/106377287#L1424

plugins/CustomVariables/Reports/GetCustomVariables.php

@@ -48,7 +48,7 @@ class GetCustomVariables extends Base
     /**
      * @return array
      */
-    protected function getFooterMessageExplanationMissingMetrics()
+    public function getFooterMessageExplanationMissingMetrics()
     {
         $metrics = sprintf("'%s', '%s' %s '%s'",
             Piwik::translate('General_ColumnNbVisits'),