MDL-41348 question code depends on filelib.

I think this used to work because mod/forum/lib.php used to be included
everywhere, and in turn included lib/formslib.php. We should declare the
dependencey explicitly.

question/engine/lib.php

@@ -26,6 +26,7 @@
 
 defined('MOODLE_INTERNAL') || die();
 
+require_once($CFG->libdir . '/filelib.php');
 require_once(dirname(__FILE__) . '/questionusage.php');
 require_once(dirname(__FILE__) . '/questionattempt.php');
 require_once(dirname(__FILE__) . '/questionattemptstep.php');