Use JFormHelper::loadFieldClass

This PR is in advance of me hopefully getting this through into J3.3:

https://github.com/joomla/joomla-cms/pull/3116

Whilst it should now autoload all the classes happily meaning they should always exist I guess it can't hurt to use the Joomla built in loader just in case somebody somewhere find somewhere it doesn't happen and then the require_once fails :)