Update to libvpx v1.8.0

[libvpx v1.8.0](https://chromium.googlesource.com/webm/libvpx/+/v1.8.0) was tagged yesterday ([GitHub](https://github.com/webmproject/libvpx/releases/tag/v1.8.0)), it would be nice if libvpx-go could be updated to this version.