ping: Code refactoring

Change-Id: I8b4e2c9dd3ba3adfae8d296a5635b048b35cf593
Refs: #3137
diff --git a/core/common.hpp b/core/common.hpp
index fa035c6..09d378c 100644
--- a/core/common.hpp
+++ b/core/common.hpp
@@ -64,6 +64,7 @@
 #include <ndn-cxx/security/signing-helpers.hpp>
 #include <ndn-cxx/security/signing-info.hpp>
 #include <ndn-cxx/util/scheduler.hpp>
+#include <ndn-cxx/util/scheduler-scoped-event-id.hpp>
 #include <ndn-cxx/util/signal.hpp>
 
 namespace ndn {