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/lib/crypto/Base64.hxx

@@ -30,7 +30,7 @@
 #ifndef BASE64_HXX
 #define BASE64_HXX
 
-#include <stddef.h>
+#include <cstddef>
 
 template<typename T> struct WritableBuffer;
 struct StringView;

src/lib/ffmpeg/Buffer.hxx

@@ -24,7 +24,7 @@ extern "C" {
 #include <libavutil/mem.h>
 }
 
-#include <stddef.h>
+#include <cstddef>
 
 /* suppress the ffmpeg compatibility macro */
 #ifdef SampleFormat

src/lib/nfs/FileReader.hxx

@@ -25,11 +25,11 @@
 #include "event/DeferEvent.hxx"
 #include "util/Compiler.h"
 
-#include <string>
+#include <cstddef>
 #include <exception>
+#include <string>
 
 #include <stdint.h>
-#include <stddef.h>
 #include <sys/stat.h>
 
 struct nfsfh;

src/lib/xiph/OggSyncState.hxx

@@ -22,7 +22,8 @@
 
 #include <ogg/ogg.h>
 
-#include <stddef.h>
+#include <cstddef>
+
 #include <stdint.h>
 
 class Reader;