net-p2p/retroshare: unbreak with ffmpeg 4.0

gui/VideoProcessor.cpp:456:40: error: use of undeclared identifier 'CODEC_CAP_TRUNCATED'

if (encoding_codec->capabilities & CODEC_CAP_TRUNCATED)
                                   ^

gui/VideoProcessor.cpp:457:36: error: use of undeclared identifier 'CODEC_FLAG_TRUNCATED'

encoding_context->flags |= CODEC_FLAG_TRUNCATED;
                           ^

gui/VideoProcessor.cpp:458:32: error: use of undeclared identifier 'CODEC_FLAG_PSNR'

encoding_context->flags |= CODEC_FLAG_PSNR;//Peak signal-to-noise ratio
                           ^

gui/VideoProcessor.cpp:459:32: error: use of undeclared identifier 'CODEC_CAP_PARAM_CHANGE'

encoding_context->flags |= CODEC_CAP_PARAM_CHANGE;
                           ^

gui/VideoProcessor.cpp:552:39: error: use of undeclared identifier 'CODEC_CAP_TRUNCATED'

if(decoding_codec->capabilities & CODEC_CAP_TRUNCATED)
                                  ^

gui/VideoProcessor.cpp:553:36: error: use of undeclared identifier 'CODEC_FLAG_TRUNCATED'

decoding_context->flags |= CODEC_FLAG_TRUNCATED; // we do not send complete frames
                           ^

gui/VideoProcessor.cpp:555:33: error: use of undeclared identifier 'CODEC_FLAG2_CHUNKS'

decoding_context->flags2 |= CODEC_FLAG2_CHUNKS;
                            ^

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