WebRTC requires a newer, unreleased, version of libvpx. The config test should
detect it once it has been released. Until then the bundled version must be
used.

Task-number: QTBUG-70183
Change-Id: Ie97319d3f9560e44ebc823fc07a481696a82648e
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>