security: add missing out-of-line destructors for transform classes

Change-Id: I8306b8e10953e55e056b4eb5fcb76ec8f395f5c4
diff --git a/src/security/transform/digest-filter.hpp b/src/security/transform/digest-filter.hpp
index 0ccc81b..a9c023e 100644
--- a/src/security/transform/digest-filter.hpp
+++ b/src/security/transform/digest-filter.hpp
@@ -1,5 +1,5 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
-/**
+/*
  * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
@@ -41,6 +41,8 @@
   explicit
   DigestFilter(DigestAlgorithm algo);
 
+  ~DigestFilter();
+
 private:
   /**
    * @brief Append data @p buf into digest calculation