Update to 2.0.8, based on PR 47467 by John D. Baker.
While here, set LICENSE.

Changes:

FFmpegThumbnailer 2.0.8
- Better ffmpeg detection
- Fix build issue with recent ffmpeg
- Optionally register ffmpegthumbnailer as gnome3 thumbnailer (--enable-thumbnailer)

version 2.0.7
- Fix build issue with recent gcc
- Fix build issue with recent ffmpeg
Updated multimedia/ffmpegthumbnailer to 2.0.8