_STD -> _VSTD to avoid macro clash on windows

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@134190 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/cstddef b/include/cstddef
index 53ef272..b851ea0 100644
--- a/include/cstddef
+++ b/include/cstddef
@@ -82,7 +82,7 @@
 
 inline _LIBCPP_ALWAYS_INLINE nullptr_t __get_nullptr_t() {return nullptr_t(0);}
 
-#define nullptr _STD::__get_nullptr_t()
+#define nullptr _VSTD::__get_nullptr_t()
 
 #endif  // _LIBCPP_HAS_NO_NULLPTR