avcodec/utils: move ff_add_cpb_side_data() to encoder code

It's only used by encoders, so move it to prevent wrong usage.

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/encode.h

@@ -73,6 +73,11 @@ int ff_encode_reordered_opaque(AVCodecContext *avctx,
 int ff_encode_encode_cb(AVCodecContext *avctx, AVPacket *avpkt,
                         AVFrame *frame, int *got_packet);
 
+/**
+ * Add a CPB properties side data to an encoding context.
+ */
+AVCPBProperties *ff_encode_add_cpb_side_data(AVCodecContext *avctx);
+
 /**
  * Rescale from sample rate to AVCodecContext.time_base.
  */