libnetlink: change rtnl_send() to take void *

Avoid having to cast buffer being sent.
diff --git a/misc/arpd.c b/misc/arpd.c
index 124d3fb..4f0021b 100644
--- a/misc/arpd.c
+++ b/misc/arpd.c
@@ -281,7 +281,7 @@
 
 	addattr_l(&req.n, sizeof(req), NDA_DST, &addr, 4);
 	addattr_l(&req.n, sizeof(req), NDA_LLADDR, lla, llalen);
-	return rtnl_send(&rth, (char*)&req, req.n.nlmsg_len) <= 0;
+	return rtnl_send(&rth, &req, req.n.nlmsg_len) <= 0;
 }
 
 void prepare_neg_entry(__u8 *ndata, __u32 stamp)