Add file_create_url to xspf_playlist_thumb

In the same way than in xspf_playlist_cck we use file_create_url to complete the url of the cck filefield files, for thumbnails we should do the same.

Using file_create_url() would allow to alter the local path in order to use CDN's, this could be done using the CDN Integration module or Pressflow. In the case of Drupal 7 that is integrated.