gsmdec: convert to new channel layout API

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/gsmdec.c

@@ -34,8 +34,8 @@
 
 static av_cold int gsm_init(AVCodecContext *avctx)
 {
-    avctx->channels       = 1;
+    av_channel_layout_uninit(&avctx->ch_layout);
-    avctx->channel_layout = AV_CH_LAYOUT_MONO;
+    avctx->ch_layout      = (AVChannelLayout)AV_CHANNEL_LAYOUT_MONO;
     if (!avctx->sample_rate)
         avctx->sample_rate = 8000;
     avctx->sample_fmt     = AV_SAMPLE_FMT_S16;