privatemsg_format_date() using current year format for last December when in January

The problem is:

  if ($timestamp > mktime(0, 0, 0, 0 /* this is the month and is 1-12, not 0-11 */, 0, date('Y'))) {
    return format_date($timestamp, 'privatemsg_current_year');
  }

mktime: http://php.net/manual/de/function.mktime.php

Please review the (pretty trivial) patch.