MDL-41754 (3) quiz statistics : need to escape core_php_time_limit

after MDL-42084 was merged into upstream, introducing
core_php_time_limit, this branch that moved a reference to
core_php_time_limit to a new namespace was broken.

lib/classes/progress/base.php

@@ -212,7 +212,7 @@ abstract class base {
         $this->lastprogresstime = $now;
 
         // Update time limit before next progress display.
-        core_php_time_limit::raise(self::TIME_LIMIT_WITHOUT_PROGRESS);
+        \core_php_time_limit::raise(self::TIME_LIMIT_WITHOUT_PROGRESS);
         $this->update_progress();
     }