multimedia/avidemux-plugins: Mark broken after multimedia/vapoursynth update (a5e4dc2)

It's been spamming pkg-fallout mailing list for a while. I think
it's also expected to be removed/deprecated soon. I am just marking
BROKEN to stop the spam for now.

Reported by: pkg-fallout
Approved by: osa, vvd (Mentors, implicit)

(cherry picked from commit 1ebcc067467ffd4f122f76b17d07426bb3c83f49)