FindFFmpeg.cmake doesn't look for correct libraries on iOS

At the time of writing, when building Qt Multimedia from source with FFmpeg support, you can get past the FindFFmpeg stage even if you don't have the correct .xcframework files in your FFMPEG_DIR.

This is because we recently switched from deploying .dylibs to deploying .xcframeworks when on iOS. The QtDeployFFmpeg.cmake script was updated, but not FindFFmpeg.cmake. This effectively means that we only really care about deploying .xcframeworks, but the FindFFmpeg.cmake script requires the .dylibs to be present in FFMPEG_DIR.

This is stopping us from switching to only relying on .xcframeworks when building.