package/gd: Add dependency to libvpx

Needed to get reproducable builds and to reproduce this build error:
http://autobuild.buildroot.net/results/046/046dd505feb5e92bdee3d0993366be162da1223a/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>