clean_ffmpeg.sh: Add missing ARM files (fix FTBFS on ARM)
    
    Also remove an unused #include of a file we do not ship from an ARM
    file. (fft_init_arm.c wants synth_filter.h, but does not actually use
    anything from it. We ship neither synth_filter.h nor synth_filter.c.)
    
    Mention the change in the qt5-qtwebengine.spec %changelog.