Slim down `common.hpp`

Change-Id: Ic9843a1b9ffc5b5b1fa6062c8250ba28c6aaa898
diff --git a/tools/ping/server/tracer.cpp b/tools/ping/server/tracer.cpp
index 0a9df40..e09c481 100644
--- a/tools/ping/server/tracer.cpp
+++ b/tools/ping/server/tracer.cpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2015-2022,  Arizona Board of Regents.
+ * Copyright (c) 2015-2024,  Arizona Board of Regents.
  *
  * This file is part of ndn-tools (Named Data Networking Essential Tools).
  * See AUTHORS.md for complete list of ndn-tools authors and contributors.
@@ -21,6 +21,8 @@
 
 #include "tracer.hpp"
 
+#include <iostream>
+
 namespace ndn::ping::server {
 
 Tracer::Tracer(PingServer& pingServer, const Options& options)