Fix C++03 build failure

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@355758 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/atomic b/include/atomic
index 2f10eab..711c78a 100644
--- a/include/atomic
+++ b/include/atomic
@@ -629,7 +629,7 @@
 
 #endif // _LIBCPP_STD_VER > 17
 
-static_assert(is_same<underlying_type<memory_order>::type, __memory_order_underlying_t>::value,
+static_assert((is_same<underlying_type<memory_order>::type, __memory_order_underlying_t>::value),
   "unexpected underlying type for std::memory_order");
 
 #if defined(_LIBCPP_HAS_GCC_ATOMIC_IMP) || \