xfce4-panel: fix QA warnings static libs in dev package

There was a patch sent already for fixing [1]. In that patch the static
libraries of panel plugins were deleted during install. Here all static libs
are packed into ${PN}-staticdev. The decision for this solution is based on:

* Keep recipes as simple as possible
* Although unlikely there might be a use case for having static libs for panel
  plugins

[1] http://lists.linuxtogo.org/pipermail/openembedded-devel/2012-February/037926.html



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