rtpdec_amr: set channel layout

libavformat/rtpdec_amr.c

@@ -19,6 +19,7 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include "libavutil/channel_layout.h"
 #include "avformat.h"
 #include "rtpdec_formats.h"
 #include "libavutil/avstring.h"
@@ -77,6 +78,7 @@ static int amr_handle_packet(AVFormatContext *ctx,
         av_log(ctx, AV_LOG_ERROR, "Only mono AMR is supported\n");
         return AVERROR_INVALIDDATA;
     }
+    st->codec->channel_layout = AV_CH_LAYOUT_MONO;
 
     /* The AMR RTP packet consists of one header byte, followed
      * by one TOC byte for each AMR frame in the packet, followed