Fix include <cstddef> only when __cpp_lib_byte is defined and sufficient

Signed-off-by: yexiaochuan <tap91624@gmail.com>

Author: XueSongTap

include/nlohmann/detail/meta/type_traits.hpp

@@ -13,8 +13,9 @@
 #include <tuple> // tuple
 #include <type_traits> // false_type, is_constructible, is_integral, is_same, true_type
 #include <utility> // declval
-#include <cstddef> // byte
-
+#if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
+    #include <cstddef> // byte
+#endif
 #include <nlohmann/detail/iterators/iterator_traits.hpp>
 #include <nlohmann/detail/macro_scope.hpp>
 #include <nlohmann/detail/meta/call_std/begin.hpp>

single_include/nlohmann/json.hpp

@@ -3367,8 +3367,9 @@ NLOHMANN_JSON_NAMESPACE_END
 #include <tuple> // tuple
 #include <type_traits> // false_type, is_constructible, is_integral, is_same, true_type
 #include <utility> // declval
-#include <cstddef> // byte
-
+#if defined(__cpp_lib_byte) && __cpp_lib_byte >= 201603L
+    #include <cstddef> // byte
+#endif
 // #include <nlohmann/detail/iterators/iterator_traits.hpp>
 //     __ _____ _____ _____
 //  __|  |   __|     |   | |  JSON for Modern C++