forward-declare struct iovec

avoids warnings of the form:

warning: 'struct iovec' declared inside parameter list
warning: its scope is only this definition or declaration, which is probably not what you want

when building downstream projects

include/zmq.h

@@ -342,6 +342,8 @@ ZMQ_EXPORT int zmq_sendmsg (void *s, zmq_msg_t *msg, int flags);
 ZMQ_EXPORT int zmq_recvmsg (void *s, zmq_msg_t *msg, int flags);
 
 /*  Experimental                                                              */
+struct iovec;
+
 ZMQ_EXPORT int zmq_sendiov (void *s, struct iovec *iov, size_t count, int flags);
 ZMQ_EXPORT int zmq_recviov (void *s, struct iovec *iov, size_t *count, int flags);