package/mesa3d: add libvdpau optional dependency

If libvdpau package is build before mesa3d, it will be detected and
enabled by the configure script.

Handle libvdpau in mesa3d for consistency.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>