Enable libwebp on ffmpeg

Summary:
WebP is a raster graphics file format developed by Google,
and spotted in a video produced on Android and uploaded to Google Photos.

This fixes the following issue:
[vost#0:0 @ 0x32d425234700] Unknown encoder 'webp'
[vost#0:0 @ 0x32d425234700] Error selecting an encoder

Test Plan:

• Rebuild ffmpeg
• identify test-video-file.mp4

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3621