fixed notice from wordpress with register_uninstall_hook

thrown notice:

Notice: register_uninstall_hook was called <strong>incorrectly</strong>. Only a static class method or function can be used in an uninstall hook. Please see <a href="http://codex.wordpress.org/Debugging_in_WordPress">Debugging in WordPress</a> for more information. (This message was added in version 3.1.) in \wordpress\wp-includes\functions.php on line 2959