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)