Fixed FFMPEG not found error

[dxbstyle]

FFmpeg doesn't support find_package in CMake, instead you have to write this for it to work

[Almamu]

There's a custom FindFFMPEG file written to take care of searching for FFMPEG: https://github.com/Almamu/linux-wallpaperengine/blob/main/CMakeModules/FindFFMPEG.cmake

If you have problems with the automatic detection of FFMPEG that's the file you should be modifying,