MDL-68647 forum: Add missing advanced grading library inclusion

The method get_grading_manager() is called in the file so make sure the
library is loaded. Without it, the forum's privacy provider unit test
ended with error.

Not directly related to the issue, but noticed while working on it.

mod/forum/classes/privacy/provider.php

@@ -37,6 +37,8 @@ use tool_dataprivacy\context_instance;
 
 defined('MOODLE_INTERNAL') || die();
 
+require_once($CFG->dirroot . '/grade/grading/lib.php');
+
 /**
  * Implementation of the privacy subsystem plugin provider for the forum activity module.
  *