Change require to require_once to stop unit tests crashing

When running the full unit test suite on a Moodle install with forumng in it, we are getting a fatal error:

> PHP Fatal error:  Cannot redeclare make_log_url() (previously declared in /var/www/jyhoam-test/course/lib.php:61) in /var/www/jyhoam-test/course/lib.php on line 142

This is because forumng is trying to include /course/lib.php for a second time. If we change the "require" to a "require_once" it fixes the problem.

Cheers

Michael