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/client/Client.hxx

@@ -31,12 +31,11 @@
 #include <boost/intrusive/link_mode.hpp>
 #include <boost/intrusive/list_hook.hpp>
 
-#include <set>
-#include <string>
+#include <cstddef>
 #include <list>
 #include <memory>
-
-#include <stddef.h>
+#include <set>
+#include <string>
 
 class SocketAddress;
 class UniqueSocketDescriptor;