Copyright for xfce4 source package in Xenial

This pseudo meta package was cobbled together by Martin Loschwitz on
July 20th, 2003.

Xfce is © 1996-2007 Olivier Fourdan 

The packaging is © The pkg-xfce debian team and is licensed under the GNU
GPL version 2 or later.

On Debian systems, the complete text of the GNU General Public 
License can be found in `/usr/share/common-licenses/GPL'.