security: code cleanup and doxygen improvements in tpm::BackEndOsx

Change-Id: I6d6d77ab315a10c280e7643d4cec5269ed10fe5e
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 692ba65..ddf629b 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) 2013-2017 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).
  *
@@ -47,7 +47,7 @@
   }
 
   std::string
-  getScheme()
+  getScheme() const
   {
     return "tpm-memory";
   }