install_helpers: add iostream include to tools

fixes compilation error on Ubuntu 18.04

Change-Id: I5227413788b9a5e1e88c195a20177939fae39281
diff --git a/install_helpers/tools/test-nack-consumer.cpp b/install_helpers/tools/test-nack-consumer.cpp
index fb6cb96..4af568e 100644
--- a/install_helpers/tools/test-nack-consumer.cpp
+++ b/install_helpers/tools/test-nack-consumer.cpp
@@ -9,6 +9,8 @@
 #include <boost/asio.hpp>
 #include <ndn-cxx/face.hpp>
 
+#include <iostream>
+
 namespace ndn {
 
 class TestNackConsumer : boost::noncopyable
@@ -222,4 +224,4 @@
   testNackConsumer.run();
 
   return 0;
-}
\ No newline at end of file
+}