ffmpegthumbs ffmpeg 8 support

[Albert Astals Cid]

It seems ffmpegthumbs does not compile with ffmpeg 8 on the release/25.08 
branch.

https://invent.kde.org/multimedia/ffmpegthumbs/-/merge_requests/38 fixes it 
but also increases the minimum ffpmeg required version so I would rather not 
commit it at this point.

But if you *know* your distribution has ffmpeg 8 and does not care about an 
older one you may want to add that patch.

Cheers,
  Albert