Avoid using deprecated ndn-cxx type aliases

Change-Id: I55fd500377698adc8035113bdff2d64091a36e07
diff --git a/src/hello-protocol.hpp b/src/hello-protocol.hpp
index c1664bd..a50c2b5 100644
--- a/src/hello-protocol.hpp
+++ b/src/hello-protocol.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2022,  The University of Memphis,
+ * Copyright (c) 2014-2023,  The University of Memphis,
  *                           Regents of the University of California,
  *                           Arizona Board of Regents.
  *
@@ -83,7 +83,7 @@
   void
   processInterest(const ndn::Name& name, const ndn::Interest& interest);
 
-  ndn::util::signal::Signal<HelloProtocol, Statistics::PacketType> hpIncrementSignal;
+  ndn::util::Signal<HelloProtocol, Statistics::PacketType> hpIncrementSignal;
 
 private:
   /*! \brief Try to contact a neighbor via Hello protocol again