Fix C++17 dylib build

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@299401 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/algorithm b/include/algorithm
index bba6d8d..571edd2 100644
--- a/include/algorithm
+++ b/include/algorithm
@@ -3026,7 +3026,8 @@
     return static_cast<result_type>(__u + __p.a());
 }
 
-#if _LIBCPP_STD_VER <= 14 || defined(_LIBCPP_ENABLE_CXX17_REMOVED_RANDOM_SHUFFLE)
+#if _LIBCPP_STD_VER <= 14 || defined(_LIBCPP_ENABLE_CXX17_REMOVED_RANDOM_SHUFFLE) \
+  || defined(_LIBCPP_BUILDING_LIBRARY)
 class _LIBCPP_TYPE_VIS __rs_default;
 
 _LIBCPP_FUNC_VIS __rs_default __rs_get();