Fixed [support.types].  <cstddef> wasn't definining NULL or offsetof.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@104982 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/cstddef b/include/cstddef
index efe0f61..8264a8b 100644
--- a/include/cstddef
+++ b/include/cstddef
@@ -34,9 +34,11 @@
 */
 
 #include <__config>
+#define __need_NULL
 #define __need_ptrdiff_t
 #define __need_size_t
 #include <stddef.h>
+#include <stddef.h>
 
 #pragma GCC system_header