[FFmpeg-user] Latest GIT version doesn't compile with libvpx_vp9 encoder

[ZikZak]

ffmpeg and libvpx are both their latest git version.

vpxenc --help:
Included encoders:

    vp8    - WebM Project VP8 Encoder v1.6.1-1317-ga924845
    vp9    - WebM Project VP9 Encoder v1.6.1-1317-ga924845 (default)

At least now VP8 encoder's back, I'll continue to update libvpx and ffmpeg
until they live happily together again :)