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/time/ISO8601.hxx

@@ -38,7 +38,7 @@
 #include <chrono>
 #include <utility>
 
-#include <stddef.h>
+#include <cstddef>
 
 struct tm;
 template<size_t CAPACITY> class StringBuffer;