graphics/vapoursynth-fmtconv: update to r24

Due to https://github.com/EleonoreMizo/fmtconv/commit/eb847d926a28
system "VapourSynth.h" from multimedia/vapoursynth cannot be used.

Changes: https://github.com/EleonoreMizo/fmtconv/releases/tag/r24
Reported by: GitHub (watch releases)