MINOR: tcp/uxst/sockpair: use fd_want_send() instead of conn_xprt_want_send()

Just like previous commit, we don't need to pass through the connection
layer anymore to enable polling during a connect(), we know the FD, so
let's simply call fd_want_send().
diff --git a/src/proto_sockpair.c b/src/proto_sockpair.c
index bdc7ebe..75cdfd4 100644
--- a/src/proto_sockpair.c
+++ b/src/proto_sockpair.c
@@ -332,7 +332,7 @@
 		return SF_ERR_RESOURCE;
 	}
 
-	conn_xprt_want_send(conn);  /* for connect status, proxy protocol or SSL */
+	fd_want_send(fd);  /* for connect status, proxy protocol or SSL */
 	return SF_ERR_NONE;  /* connection is OK */
 }