encoding+lp: declare constexpr and noexcept where possible

Change-Id: Icf708af0b77d62d7d8e75527aaf51c7178e47125
diff --git a/src/lp/packet.hpp b/src/lp/packet.hpp
index eb30485..1aba8b1 100644
--- a/src/lp/packet.hpp
+++ b/src/lp/packet.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2017 Regents of the University of California.
+ * Copyright (c) 2013-2018 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -204,7 +204,7 @@
 
 private:
   static bool
-  comparePos(uint64_t first, const Block& second);
+  comparePos(uint64_t first, const Block& second) noexcept;
 
 private:
   mutable Block m_wire;