MDL-65694 grade: Fix missing gradelib library

In the be8f453b74 the gradelib loading was removed by mistake. So
the library may not be loaded and executing the task may end with
the exception - class 'grade_grade' not found.

lib/classes/task/grade_cron_task.php

@@ -43,6 +43,7 @@ class grade_cron_task extends scheduled_task {
      */
     public function execute() {
         global $CFG, $DB;
+        require_once($CFG->libdir . '/gradelib.php');
 
         $now = time();
         $sql = "SELECT i.*