COPYING file license does not match code headers

The xfce4-xkb-plugin file COPYING seems to contain boilerplate text for a BSD-2 license, while the code headers indicate that this is released under a GPLv2 license.

Please update the COPYING file to be consistent with how the individual code is licensed. See https://opensource.org/licenses/gpl-2.0.php for reference or other Xfce component COPYING files.