replace stddef.h with cstddef

The former is deprecated with C++14. The standard says both are the same:

The contents and meaning of the header<cstddef>are the same as the C
standard library header<stddef.h>,except that it does not declare the type
wchar_t, that it also declares the type byte and its associated
operations (21.2.5), and as noted in 21.2.3 and 21.2.4.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/pcm/AudioFormat.hxx

@@ -24,10 +24,9 @@
 #include "pcm/ChannelDefs.hxx"
 #include "util/Compiler.h"
 
+#include <cstddef>
 #include <cstdint>
 
-#include <stddef.h>
-
 template<size_t CAPACITY> class StringBuffer;
 
 /**

src/pcm/Dsd2Pcm.hxx

@@ -34,8 +34,8 @@ or implied, of Sebastian Gesemann.
 #include "ChannelDefs.hxx"
 
 #include <array>
+#include <cstddef>
 
-#include <stddef.h>
 #include <stdint.h>
 
 /**

src/pcm/Mix.hxx

@@ -23,7 +23,7 @@
 #include "SampleFormat.hxx"
 #include "util/Compiler.h"
 
-#include <stddef.h>
+#include <cstddef>
 
 class PcmDither;
 

src/pcm/RestBuffer.hxx

@@ -24,8 +24,7 @@
 
 #include <algorithm>
 #include <cassert>
-
-#include <stddef.h>
+#include <cstddef>
 
 template<typename T> struct ConstBuffer;
 class PcmBuffer;

src/pcm/Traits.hxx

@@ -22,8 +22,9 @@
 
 #include "SampleFormat.hxx"
 
+#include <cstddef>
+
 #include <stdint.h>
-#include <stddef.h>
 
 /**
  * This template describes the specified #SampleFormat.  This is an