Fix compatibility with FFmpeg 8

[hannesbraun]

The av_rdft_* functions are no longer available with FFmpeg 8. This patch adds compatibility with FFmpeg 8. The build should also still work with older versions of FFmpeg.

[nunotexbsd]

Tested OK on FreeBSD with FFmpeg 8

(Also OK with FFmpeg 6)

[jakeobsen]

This is tested with my ARM build for MacOS - see #345. Spek performs as expected 👍

$ ffmpeg
ffmpeg version 8.1 Copyright (c) 2000-2026 the FFmpeg developers
  built with Apple clang version 21.0.0 (clang-2100.0.123.102)
  configuration: --prefix=/opt/homebrew/Cellar/ffmpeg/8.1 --enable-shared --enable-pthreads --enable-version3 --cc=clang --host-cflags= --host-ldflags= --enable-ffplay --enable-gpl --enable-libsvtav1 --enable-libopus --enable-libx264 --enable-libmp3lame --enable-libdav1d --enable-libvmaf --enable-libvpx --enable-libx265 --enable-openssl --enable-videotoolbox --enable-audiotoolbox --enable-neon
  libavutil      60. 26.100 / 60. 26.100
  libavcodec     62. 28.100 / 62. 28.100
  libavformat    62. 12.100 / 62. 12.100
  libavdevice    62.  3.100 / 62.  3.100
  libavfilter    11. 14.100 / 11. 14.100
  libswscale      9.  5.100 /  9.  5.100
  libswresample   6.  3.100 /  6.  3.100
Universal media converter
usage: ffmpeg [options] [[infile options] -i infile]... {[outfile options] outfile}...

Use -h to get full help or, even better, run 'man ffmpeg'