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

Change-Id: I7ed2893acbda1458383603c0e3c154d3e841405e
Refs: #3560
diff --git a/model/ndn-l3-protocol.cpp b/model/ndn-l3-protocol.cpp
index a3263f4..8556db7 100644
--- a/model/ndn-l3-protocol.cpp
+++ b/model/ndn-l3-protocol.cpp
@@ -233,6 +233,12 @@
 };
 
 void
+L3Protocol::injectInterest(const Interest& interest)
+{
+  m_impl->m_internalFace->sendInterest(interest);
+}
+
+void
 L3Protocol::initializeManagement()
 {
   auto& forwarder = m_impl->m_forwarder;