vp9_svc_layercontext: Clean up function definition mismatch.

vp9_is_upper_layer_key_frame() definition does not match declaration--
it was missing the second const.

Change-Id: I71312579eb443be1924b8b06d8b3177c3dcb40f3

vp9/encoder/vp9_svc_layercontext.c

@@ -221,7 +221,7 @@ void vp9_inc_frame_in_layer(SVC *svc) {
   ++lc->current_video_frame_in_layer;
 }
 
-int vp9_is_upper_layer_key_frame(const VP9_COMP *cpi) {
+int vp9_is_upper_layer_key_frame(const VP9_COMP *const cpi) {
   return cpi->use_svc &&
          cpi->svc.number_temporal_layers == 1 &&
          cpi->svc.spatial_layer_id > 0 &&