security+util: mark all pimpl unique_ptrs const

Change-Id: I3966c4988dceb49c204e23967ba90729877bddf1
diff --git a/tests/unit-tests/security/tpm/back-end-wrapper-mem.hpp b/tests/unit-tests/security/tpm/back-end-wrapper-mem.hpp
index ea10295..692ba65 100644
--- a/tests/unit-tests/security/tpm/back-end-wrapper-mem.hpp
+++ b/tests/unit-tests/security/tpm/back-end-wrapper-mem.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2016 Regents of the University of California.
+ * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -36,8 +36,8 @@
 {
 public:
   BackEndWrapperMem()
+    : m_impl(make_unique<BackEndMem>())
   {
-    m_impl = unique_ptr<BackEnd>(new BackEndMem);
   }
 
   BackEnd&
@@ -53,7 +53,7 @@
   }
 
 private:
-  unique_ptr<BackEnd> m_impl;
+  const unique_ptr<BackEnd> m_impl;
 };
 
 } // namespace tests