Fix compilation with Boost 1.65.0

Change-Id: I6f56d6bf135d9c862b04cb086340301ee98ef0ed
Refs: #4256
diff --git a/tests/integrated/scheduler-benchmark.cpp b/tests/integrated/scheduler-benchmark.cpp
index 3074d22..5fa57b7 100644
--- a/tests/integrated/scheduler-benchmark.cpp
+++ b/tests/integrated/scheduler-benchmark.cpp
@@ -28,6 +28,8 @@
 #include "boost-test.hpp"
 #include "timed-execute.hpp"
 
+#include <iostream>
+
 namespace ndn {
 namespace util {
 namespace scheduler {