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/istream b/include/istream
index 24d9f3a..f271393 100644
--- a/include/istream
+++ b/include/istream
@@ -155,6 +155,8 @@
 #include <__config>
 #include <ostream>
 
+#include <__undef_min_max>
+
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
 #endif