directfb-examples: install rose.png for df_dok

The directfb example program df_dok now requires rose.png to be
installed, otherwise it gives the following error:

(#) DirectFBError [dfb->CreateImageProvider( dfb, DATADIR"/rose.png", &provider )]: A general or unknown error occured

Therefore add rose.png to the list of data files if df_dok is enabled.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>