zmq_msg_t will now typedef a named struct.

This change allows forward declaration of struct zmq_msg_t.

include/zmq.h

@@ -176,7 +176,7 @@ ZMQ_EXPORT int zmq_term (void *context);
 /*  0MQ message definition.                                                   */
 /******************************************************************************/
 
-typedef struct {unsigned char _ [32];} zmq_msg_t;
+typedef struct zmq_msg_t {unsigned char _ [32];} zmq_msg_t;
 
 typedef void (zmq_free_fn) (void *data, void *hint);