xfce4-diskperf-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 About function) makes it clear that a BSD
license is intended:

      "license", xfce_get_license_text (XFCE_LICENSE_TEXT_BSD),

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