model: Add method that is necessary for python bindings

Refs: #2341
diff --git a/model/ndn-l3-protocol.hpp b/model/ndn-l3-protocol.hpp
index 11de1b8..79583c1 100644
--- a/model/ndn-l3-protocol.hpp
+++ b/model/ndn-l3-protocol.hpp
@@ -151,6 +151,10 @@
   nfd::ConfigSection&
   getConfig();
 
+public: // Workaround for python bindings
+  static Ptr<L3Protocol>
+  getL3Protocol(Ptr<Object> node);
+
 protected:
   virtual void
   DoDispose(void); ///< @brief Do cleanup