Set channel_layout for mpegaudio

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>
(cherry picked from commit f4a86bc9)

libavcodec/mpegaudiodec.c

@@ -2054,6 +2054,7 @@ static int decode_frame(AVCodecContext * avctx,
     }
     /* update codec info */
     avctx->channels = s->nb_channels;
+    avctx->channel_layout = s->nb_channels == 1 ? AV_CH_LAYOUT_MONO : AV_CH_LAYOUT_STEREO;
     if (!avctx->bit_rate)
         avctx->bit_rate = s->bit_rate;
     avctx->sub_id = s->layer;