helper+model: Fix helpers to work with NFD/ndn-cxx 0.4+

Change-Id: I7ed2893acbda1458383603c0e3c154d3e841405e
Refs: #3560
diff --git a/model/ndn-l3-protocol.hpp b/model/ndn-l3-protocol.hpp
index 0145d72..a34d36f 100644
--- a/model/ndn-l3-protocol.hpp
+++ b/model/ndn-l3-protocol.hpp
@@ -147,6 +147,12 @@
   nfd::ConfigSection&
   getConfig();
 
+  /**
+   * \brief Inject interest through internal Face
+   */
+  void
+  injectInterest(const Interest& interest);
+
 public: // Workaround for python bindings
   static Ptr<L3Protocol>
   getL3Protocol(Ptr<Object> node);