Fix unused variable error.

  CXX    libzmq_la-stream_engine.lo
stream_engine.cpp: In member function 'virtual void zmq::stream_engine_t::plug(zmq::io_thread_t*, zmq::session_base_t*)':
stream_engine.cpp:185:9: error: 'rc' was not declared in this scope
make[3]: *** [libzmq_la-stream_engine.lo] Error 1