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/decoder/DecoderBuffer.hxx

@@ -24,7 +24,8 @@
 #include "util/DynamicFifoBuffer.hxx"
 #include "util/ConstBuffer.hxx"
 
-#include <stddef.h>
+#include <cstddef>
+
 #include <stdint.h>
 
 class DecoderClient;

src/decoder/plugins/OpusHead.hxx

@@ -20,7 +20,7 @@
 #ifndef MPD_OPUS_HEAD_HXX
 #define MPD_OPUS_HEAD_HXX
 
-#include <stddef.h>
+#include <cstddef>
 
 bool
 ScanOpusHeader(const void *data, size_t size, unsigned &channels_r);

src/decoder/plugins/OpusTags.hxx

@@ -20,7 +20,7 @@
 #ifndef MPD_OPUS_TAGS_HXX
 #define MPD_OPUS_TAGS_HXX
 
-#include <stddef.h>
+#include <cstddef>
 
 struct ReplayGainInfo;
 class TagHandler;