face: Mark Face class as noncopyable

Copy of the Face object has undefined behavior and thus is prohibited.

Change-Id: I701e208c498d323c33d2a32bd6f1e163415b25a6
refs: #1252
diff --git a/src/face.hpp b/src/face.hpp
index 9b1fbe4..b617e42 100644
--- a/src/face.hpp
+++ b/src/face.hpp
@@ -50,7 +50,8 @@
 /**
  * @brief Abstraction to communicate with local or remote NDN forwarder
  */
-class Face {
+class Face : noncopyable
+{
 public:
   struct Error : public std::runtime_error { Error(const std::string &what) : std::runtime_error(what) {} };