pcm_utils: moved conversion code to pcm_convert.c

All what's left in pcm_utils.h is the pcm_range() utility function,
which is only used internally by pcm_volume and pcm_mix.

src/decoder_internal.h

@@ -20,7 +20,7 @@
 #define MPD_DECODER_INTERNAL_H
 
 #include "decoder_api.h"
-#include "pcm_utils.h"
+#include "pcm_convert.h"
 
 struct decoder {
 	struct pcm_convert_state conv_state;