libvpx: needs thread support

Fixes:

  http://autobuild.buildroot.org/results/8de/8def7e54e4fecb7df350443824c00b53d1c61c01/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: Samuel Martin <s.martin49@gmail.com>