build: switch to C++14

Cleanup core/common.hpp

Change-Id: I6566b0224e24716691d5896462aa7613547ed950
Refs: #3076

tools/ping/server/ping-server.cpp

@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
-/**
- * Copyright (c) 2015,  Arizona Board of Regents.
+/*
+ * Copyright (c) 2015-2018,  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.
@@ -22,6 +22,8 @@
 
 #include "ping-server.hpp"
 
+#include <ndn-cxx/security/signing-helpers.hpp>
+
 namespace ndn {
 namespace ping {
 namespace server {