Problem: can't set IPV6_V6ONLY on OpenBSD

Solution: skip setsockopt call resulting in EINVAL

src/ip.cpp

@@ -108,7 +108,7 @@ void zmq::enable_ipv4_mapping (fd_t s_)
 {
   (void) s_;
 
-#ifdef IPV6_V6ONLY
+#if defined IPV6_V6ONLY && !defined ZMQ_HAVE_OPENBSD
 #ifdef ZMQ_HAVE_WINDOWS
     DWORD flag = 0;
 #else