math/octave-forge-video: unbreak with ffmpeg 4.0

AVHandler.cc:246:29: error: use of undeclared identifier 'CODEC_CAP_TRUNCATED'

if (codec->capabilities & CODEC_CAP_TRUNCATED)
                          ^

AVHandler.cc:247:30: error: use of undeclared identifier 'CODEC_FLAG_TRUNCATED'

vstream->codec->flags |= CODEC_FLAG_TRUNCATED;
                         ^

PR: 227726
Reported by: antoine (via exp-run)