Add protection from min/max macros

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@145407 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/string b/include/string
index 5f5a347..344bb55 100644
--- a/include/string
+++ b/include/string
@@ -446,6 +446,8 @@
 #include <cassert>
 #endif
 
+#include <__undef_min_max>
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif