Fix error message: Strict (2048) Declaration of PhpThumbHelper::url()...

I had an error message in cakePHP 2.2.2:
Strict (2048): Declaration of PhpThumbHelper::url() should be compatible with Helper::url($url = NULL, $full = false) [APP/Plugin/PhpThumb/View/Helper/PhpThumbHelper.php, line 5]

I fixed it