css file links don't use dummy query string to force cache reloads

The normal css system in Drupal 6 appends a dummy query string to each css link to force a client-side reload of the file after each cache flush.
The print module has its own code for inserting css files that doesn't take advantage of this mechanism. This may cause a client to see an outdated version of the printed page because an old copy of the css may still be in the browser's cache.
The attached patch adds this cache buster query string to print.css inserted by the print module to ensure up to date css on the client side without requiring an explicit refresh.