xfce-panel-plugin.bbclass: pack modules also from ${libdir}/xfce4/panel-plugins

* some panel-plugins (e.g xfce4-datetime-plugin) install their modules to
  ${libdir}/xfce4/panel-plugins
* build tested with fresh build dir
* run tested on overo: applications / all available plugins

Signed-off-by: Andreas Müller <schnitzeltony@gmx.de>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>