[third_party/ffmpeg] add .gitignore and remove two .pyc files

Change-Id: I0a0d4aca0575545137346055f79cad6c8ccdaef6
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/ffmpeg/+/737596
Reviewed-by: Petr Hosek <phosek@google.com>
3 files changed