GitLab

Summary:
Preparation for updating the build process of ffmpeg-related feature
to support Windows.

Checking-in FindFFMPEG.cmake from Kitware/VTK repo without modification
so that later it's easy to follow our modification on it with git tool.

Pull Request resolved: https://github.com/pytorch/audio/pull/2125

Reviewed By: carolineechen

Differential Revision: D33405408

Pulled By: mthrok

fbshipit-source-id: 5faea8940d2dfcf0b2f647eda3754f713d21fcd1