contrib: bump vpx

Use latest libvpx (was already the case on Android),
to profit from latest optimizations.

This should mostly profit to low-end Intel CPUs,
according to libvpx changelog:

Devices based on Celeron processors have seen substantial
decoding improvements (...) While Celeron benefited most,
Core-i5 also improved 5% and 10%.

https://github.com/webmproject/libvpx/blob/master/CHANGELOG



Change-Id: I471408c2b09d580fd564b3a968eb4e0f22599541
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>