Give the emulated nullptr_t a default constructor.

llvm-svn: 150893
diff --git a/libcxx/include/cstddef b/libcxx/include/cstddef
index 48317ba..4a6b16e 100644
--- a/libcxx/include/cstddef
+++ b/libcxx/include/cstddef
@@ -62,6 +62,7 @@
 
     struct __nat {int __for_bool_;};
 
+    _LIBCPP_ALWAYS_INLINE nullptr_t() {}
     _LIBCPP_ALWAYS_INLINE nullptr_t(int __nat::*) {}
 
     _LIBCPP_ALWAYS_INLINE operator int __nat::*() const {return 0;}