Always build in C++11 mode.

Change-Id: Ia25c738f63aee1b70e61f841ff52a151446ac220
Refs: #1930
diff --git a/src/util/ethernet.cpp b/src/util/ethernet.cpp
index d5eb780..f68a27a 100644
--- a/src/util/ethernet.cpp
+++ b/src/util/ethernet.cpp
@@ -28,6 +28,7 @@
 #include "ethernet.hpp"
 
 #include <stdio.h>
+#include <ostream>
 
 namespace ndn {
 namespace util {