Bump ffmpeg-source from dcc9454 to b6bb6b9

Bumps [ffmpeg-source](https://github.com/FFmpeg/FFmpeg) from `dcc9454` to `b6bb6b9`.
- [Release notes](https://github.com/FFmpeg/FFmpeg/releases)
- [Commits](FFmpeg/FFmpeg@dcc9454...b6bb6b9)

---
updated-dependencies:
- dependency-name: ffmpeg-source
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>