libavcodec/avfft.h: removed in FFmpeg 8.0

[oysstu]

Hitting a compile error on libavcodec/avfft.h file not found.

rust-ffmpeg-sys/build.rs

Line 1568 in 36eca19

let avfft_path = search_include(&include_paths, "libavcodec/avfft.h");

Deprecation notice:

2023-09-01 - 139e54911c8 - lavc 60.25.100 - avfft.h
The entire header will be deprecated and removed in two major bumps.
For a replacement to av_dct, av_rdft, av_fft and av_mdct, use
the new API from libavutil/tx.h.

[gaurav02081]

hey raised the pr for it , It basically makes sure the crate builds cleanly with FFmpeg 8 since that header got removed.