Added lib/lessphp to the list of excluded dirs

Hi guys,

https://tracker.moodle.org/browse/MDL-44357 has been integrated which added a new 3rd party lib lessphp (https://github.com/oyejorge/less.php) to Moodle.
This commit simply adds it to the list of directories to exclude as it breaks our whitespace count.

Cheers
Sam