emulators/mgba: unbreak with ffmpeg 4.0

src/feature/ffmpeg/ffmpeg-encoder.c:232:29: error: use of undeclared identifier 'CODEC_FLAG_GLOBAL_HEADER'

encoder->audio->flags |= CODEC_FLAG_GLOBAL_HEADER;
                         ^

src/feature/ffmpeg/ffmpeg-encoder.c:294:28: error: use of undeclared identifier 'CODEC_FLAG_GLOBAL_HEADER'

encoder->video->flags |= CODEC_FLAG_GLOBAL_HEADER;
                         ^

PR: 227726
Reported by: antoine (via exp-run)
Obtained from: upstream (mgba 0.6.2)