xfce4-wavelan-plugin: Specify BSD license

This software contains an older or slightly modified BSD license in
its COPYING file and assigns copyright to the specific authors.  The
code (see the wavelan_show_about function) makes it clear that a BSD
license is intended:

      "license", xfce_get_license_text (XFCE_LICENSE_TEXT_BSD),

(caveat: and of course, INAL)

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Acked-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>