qt5.inc: Fix packaging of plugins

The qtmultimedia module install the plugins in a sub-directory however
those were not being packaged. This patch fixes following QA error:

ERROR: QA Issue: qtmultimedia: Files/directories were installed but not shipped
  /usr/lib/qt5/plugins/video
  /usr/lib/qt5/plugins/video/videonode
  /usr/lib/qt5/plugins/video/videonode/libimx6vivantevideonode.so
  /usr/lib/qt5/plugins/video/videonode/.debug
  /usr/lib/qt5/plugins/video/videonode/.debug/libimx6vivantevideonode.so

Reported-by: Lauren Post <lauren.post@freescale.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>