install_helpers: add iostream include to tools

fixes compilation error on Ubuntu 18.04

Change-Id: I5227413788b9a5e1e88c195a20177939fae39281
diff --git a/install_helpers/tools/test-congestionmark-consumer.cpp b/install_helpers/tools/test-congestionmark-consumer.cpp
index fb78312..600133d 100644
--- a/install_helpers/tools/test-congestionmark-consumer.cpp
+++ b/install_helpers/tools/test-congestionmark-consumer.cpp
@@ -14,6 +14,8 @@
 #include <ndn-cxx/interest.hpp>
 #include <ndn-cxx/lp/nack.hpp>
 
+#include <iostream>
+
 using namespace ndn;
 
 class TestCongestionMarkConsumer