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

[Dysl3xik]

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.

[nilsmelchert]

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

[Dysl3xik]

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.