Add protection from min/max macros

llvm-svn: 145407
diff --git a/libcxx/include/string b/libcxx/include/string
index 5f5a347..344bb55 100644
--- a/libcxx/include/string
+++ b/libcxx/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