cstddef: Add versioned namespace.

2018-10-24  François Dumont  <fdumont@gcc.gnu.org>

	* include/c_global/cstddef: Add versioned namespace.

From-SVN: r265450

libstdc++-v3/ChangeLog

+2018-10-24  François Dumont  <fdumont@gcc.gnu.org>
+
+	* include/c_global/cstddef: Add versioned namespace.
+
 2018-10-23  Jonathan Wakely  <jwakely@redhat.com>
 
 	PR libstdc++/87704

libstdc++-v3/include/c_global/cstddef

@@ -62,6 +62,7 @@ namespace std
 #if __cplusplus >= 201703L
 namespace std
 {
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
 #define __cpp_lib_byte 201603
 
   /// std::byte
@@ -187,6 +188,7 @@ namespace std
     to_integer(__byte_op_t<_IntegerType> __b) noexcept
     { return _IntegerType(__b); }
 
+_GLIBCXX_END_NAMESPACE_VERSION
 } // namespace std
 #endif // C++17
 } // extern "C++"