SOCKET no longer defined since winsock2.h removed from zmq.h

If you try to build this library with a newer release of ZeroMQ, on a windows box, you get errors because SOCKET can't be resolved in compatability.h.

This was introduced in this merge I believe: zeromq/libzmq#3681

I will submit a PR that introduces the same fix into this code base as was proposed in that one.

Thanks for the PR in advance. Did you manage to compile zmqpp on windows?

It worked fine with my fix applied, I didn't re-pull after it was merged though, but I would assume it was fine and this can be closed.