Problem: PGM/NORM builds broken due to i_engine API change

Solution: implement get_endpoint in the NORM and PGM engines too

src/pgm_receiver.hpp

@@ -64,6 +64,7 @@ namespace zmq
         void restart_input ();
         void restart_output ();
         void zap_msg_available () {}
+        const char *get_endpoint () const;
 
         //  i_poll_events interface implementation.
         void in_event ();