Move fix_ffmpeg_dependencies.sh script from "linux" to "shared"
directory.
The script will be used on macOS but also on Windows (using MSYS2)
Pick-to: 6.8
Task-number: QTBUG-122010
Change-Id: If63d5e3ba7562b230cce26c7bfba728a2ae0a382
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>