net-im/mtxclient: unbreak with boost 1.70

lib/http/session.cpp:60:22: error: no member named 'cout' in namespace 'std'

std::cout << "shutdown: " << ec.message() << std::endl;
~~~~~^

lib/http/session.cpp:155:22: error: no member named 'cerr' in namespace 'std'

std::cerr << ec.message() << "\n";
~~~~~^