CDECL calling convention is not explicitly specified in zmq.h

[mvidelgauz]

If a client application is built with "stdcall" calling convention and links with ZMQ it gets "unresolved external symbols" errors (at least in Visual Studio)

The problem can be solved by providing explicit "cdecl" specification in all external ZMQ functions prototypes in "zmq.h"

(actually, that's the way the problem is solved in our stdcall-compiled project - by manual editing "zmq.h")