management+util: add 'override' specifier where applicable

Change-Id: Iafb9c2c59026e4268522120f405585ba35fac248
diff --git a/src/util/time-custom-clock.hpp b/src/util/time-custom-clock.hpp
index 066d3e9..57579e5 100644
--- a/src/util/time-custom-clock.hpp
+++ b/src/util/time-custom-clock.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2013-2014 Regents of the University of California.
+ * Copyright (c) 2013-2016 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -37,9 +37,8 @@
 class CustomClock
 {
 public:
-  virtual ~CustomClock()
-  {
-  }
+  virtual
+  ~CustomClock() = default;
 
   virtual typename BaseClock::time_point
   getNow() const = 0;