multimedia/simplescreenrecorder: unbreak with ffmpeg 4.0

src/AV/Output/AudioEncoder.cpp:61:51: error: use of undeclared identifier 'CODEC_CAP_VARIABLE_FRAME_SIZE'

return (GetCodecContext()->codec->capabilities & CODEC_CAP_VARIABLE_FRAME_SIZE)? DEFAULT_FRAME_SAMPLES : GetCodecContext()->frame_size;
                                                 ^

src/AV/Output/AudioEncoder.cpp:125:28: error: use of undeclared identifier 'CODEC_FLAG_QSCALE'

codec_context->flags |= CODEC_FLAG_QSCALE;
                        ^

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