Backport make_unique

C++14 has std::make_unique, which is convenient for working with unique_ptrs.

It is sufficient to backport only the overload for non-array types, the
overload for array types isn't used in ndn-cxx.

Change-Id: I200ba24ace6ed25d695f0ebe5e9b459f5dfc7538
Refs: #3093
diff --git a/src/common.hpp b/src/common.hpp
index 6a45506..8212a7e 100644
--- a/src/common.hpp
+++ b/src/common.hpp
@@ -73,6 +73,7 @@
 #include <string>
 #include <type_traits>
 #include <unistd.h>
+#include <utility>
 
 #if defined(__GNUC__) || defined(__clang__)
 #  define DEPRECATED(func) func __attribute__ ((deprecated))
@@ -137,4 +138,6 @@
 using boost::noncopyable;
 }
 
+#include "util/backports.hpp"
+
 #endif // NDN_COMMON_HPP