xfce4-xkb-plugin: Specify GPLv2 license

Like several other xfce plugins, this software contains an older or
slightly modified BSD license in its COPYING file and assigns copyright
to the specific authors.  The code itself (see the xfce_xkb_about
function) however makes it clear that a GPL license is intended:

    xfce_get_license_text (XFCE_LICENSE_TEXT_GPL));

Use GPLv2, not the author's name, as the LICENSE matching other plugins
with the same license text.

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>