graphics/gegl: unbreak with ffmpeg 4.0

./ff-load.c:312:36: error: use of undeclared identifier 'CODEC_CAP_TRUNCATED'

if (p->codec->capabilities & CODEC_CAP_TRUNCATED)
                             ^

./ff-load.c:313:26: error: use of undeclared identifier 'CODEC_FLAG_TRUNCATED'

p->enc->flags |= CODEC_FLAG_TRUNCATED;

PR: 227726