MDL-43960 assign: Add missing include

There was a report of mod_assign_save_grade webservice failing because of this.
I couldn't reproduce it, but grepping the code found one place where this include
should be added.

mod/assign/locallib.php

@@ -4066,7 +4066,9 @@ class assign {
      * @return bool
      */
     protected function gradebook_item_update($submission=null, $grade=null) {
+        global $CFG;
 
+        require_once($CFG->dirroot.'/mod/assign/lib.php');
         // Do not push grade to gradebook if blind marking is active as
         // the gradebook would reveal the students.
         if ($this->is_blind_marking()) {