audio/aubio: unbreak with ffmpeg 4.0

../src/io/source_avcodec.c:262:35: error: use of undeclared identifier 'FF_MIN_BUFFER_SIZE'

s->output = (smpl_t *)av_malloc(AUBIO_AVCODEC_MAX_BUFFER_SIZE * sizeof(smpl_t));
                                ^

../src/io/source_avcodec.c:61:39: note: expanded from macro 'AUBIO_AVCODEC_MAX_BUFFER_SIZE'
#define AUBIO_AVCODEC_MAX_BUFFER_SIZE FF_MIN_BUFFER_SIZE

^

../src/io/source_avcodec.c:360:25: error: use of undeclared identifier 'FF_MIN_BUFFER_SIZE'

int max_out_samples = AUBIO_AVCODEC_MAX_BUFFER_SIZE / avCodecCtx->channels;
                      ^

../src/io/source_avcodec.c:61:39: note: expanded from macro 'AUBIO_AVCODEC_MAX_BUFFER_SIZE'
#define AUBIO_AVCODEC_MAX_BUFFER_SIZE FF_MIN_BUFFER_SIZE

^

../src/io/source_avcodec.c:434:21: error: use of undeclared identifier 'FF_MIN_BUFFER_SIZE'

max_out_samples = AUBIO_AVCODEC_MAX_BUFFER_SIZE / avCodecCtx->channels;
                  ^

../src/io/source_avcodec.c:61:39: note: expanded from macro 'AUBIO_AVCODEC_MAX_BUFFER_SIZE'
#define AUBIO_AVCODEC_MAX_BUFFER_SIZE FF_MIN_BUFFER_SIZE

^

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