model: Enabling face operations based on NFD's face

Refs: #2215
diff --git a/model/ndn-global-router.hpp b/model/ndn-global-router.hpp
index 74cf5f1..0465cb8 100644
--- a/model/ndn-global-router.hpp
+++ b/model/ndn-global-router.hpp
@@ -22,6 +22,7 @@
 #define NDN_GLOBAL_ROUTER_H
 
 #include "ns3/ndnSIM/model/ndn-common.hpp"
+#include "ns3/ndnSIM/model/ndn-face.hpp"
 
 #include "ns3/object.h"
 #include "ns3/ptr.h"
@@ -36,7 +37,6 @@
 namespace ndn {
 
 class L3Protocol;
-class Face;
 
 /**
  * @ingroup ndn