net/freerdp: unbreak with ffmpeg 4.0

channels/tsmf/client/ffmpeg/tsmf_ffmpeg.c:210:38: error: use of undeclared identifier 'CODEC_CAP_TRUNCATED'

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

channels/tsmf/client/ffmpeg/tsmf_ffmpeg.c:211:37: error: use of undeclared identifier 'CODEC_FLAG_TRUNCATED'

mdecoder->codec_context->flags |= CODEC_FLAG_TRUNCATED;
                                  ^

PR: 227726
Reported by: antoine (via exp-run)
Obtained from: upstream (FreeRDP 2.0.0-rc2)