common: drop inclusion of unistd.h

Change-Id: I1d2a5038f1ecf568eb7ba5cbab28c04c7ca62199
diff --git a/ndn-cxx/detail/common.hpp b/ndn-cxx/detail/common.hpp
index 7fa4bcf..f5c95e6 100644
--- a/ndn-cxx/detail/common.hpp
+++ b/ndn-cxx/detail/common.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2013-2018 Regents of the University of California.
+ * Copyright (c) 2013-2019 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
  *
@@ -59,7 +59,6 @@
 #include <stdexcept>
 #include <string>
 #include <type_traits>
-#include <unistd.h>
 #include <utility>
 
 namespace ndn {