benchmarks/postal: Unbreak on CURRENT

tcp.cpp:135:10: error: assigning to 'int' from incompatible type '__bind<int &, sockaddr *&, unsigned long>'

rc = bind(m_fd, source, sizeof(struct sockaddr_in));
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p500844_s347134/logs/errors/postal-0.73_3.log