Fix return code redeclaration for platforms with SO_NOSIGPIPE

Fixes :

stream_engine.cpp: In constructor 'zmq::stream_engine_t::stream_engine_t(zmq::fd_t, const zmq::options_t&, const std::string&)':
stream_engine.cpp:102: error: redeclaration of 'int rc'
stream_engine.cpp:73: error: 'int rc' previously declared here