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/fs/Limits.hxx

@@ -21,8 +21,7 @@
 #define MPD_FS_LIMITS_HXX
 
 #include <climits>
-
-#include <stddef.h>
+#include <cstddef>
 
 #if defined(_WIN32)
 static constexpr size_t MPD_PATH_MAX = 260;

src/fs/io/BufferedOutputStream.hxx

@@ -33,7 +33,7 @@
 #include "util/Compiler.h"
 #include "util/DynamicFifoBuffer.hxx"
 
-#include <stddef.h>
+#include <cstddef>
 
 #ifdef _UNICODE
 #include <wchar.h>

src/fs/io/BufferedReader.hxx

@@ -33,7 +33,7 @@
 #include "util/Compiler.h"
 #include "util/DynamicFifoBuffer.hxx"
 
-#include <stddef.h>
+#include <cstddef>
 
 class Reader;
 

src/fs/io/OutputStream.hxx

@@ -30,7 +30,7 @@
 #ifndef OUTPUT_STREAM_HXX
 #define OUTPUT_STREAM_HXX
 
-#include <stddef.h>
+#include <cstddef>
 
 class OutputStream {
 public:

src/fs/io/Reader.hxx

@@ -32,7 +32,7 @@
 
 #include "util/Compiler.h"
 
-#include <stddef.h>
+#include <cstddef>
 
 /**
  * An interface that can read bytes from a stream until the stream