pcm: Add const to cast in ENCODE macro.

This fixes a lot of warnings of the sort:
libavcodec/pcm.c:105: warning: cast discards qualifiers from pointer target type

libavcodec/pcm.c

@@ -71,7 +71,7 @@ static av_cold int pcm_encode_close(AVCodecContext *avctx)
  * @param offset Sample value offset
  */
 #define ENCODE(type, endian, src, dst, n, shift, offset) \
-    samples_##type = (type*)src; \
+    samples_##type = (const type*) src; \
     for(;n>0;n--) { \
         register type v = (*samples_##type++ >> shift) + offset; \
         bytestream_put_##endian(&dst, v); \