route: Fix a unused-but-set-variable warning on OpenBSD

So we could enable -Werror for OpenBSD builds.

Change-Id: Ic971604beb1320d7b9d6121cd8e8519ccc1a7eb9
Signed-off-by: Frank Lichtenheld <[email protected]>
Acked-by: MaxF <[email protected]>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1214
Message-Id: <[email protected]>
URL: https://sourceforge.net/p/openvpn/mailman/message/59242246/
Signed-off-by: Gert Doering <[email protected]>

Author: flichtenheld

src/openvpn/route.c

@@ -2221,6 +2221,7 @@ delete_route_ipv6(const struct route_ipv6 *r6, const struct tuntap *tt, const st
         device = r6->iface;
         gateway_needed = true;
     }
+    (void)device; /* unused on some platforms */
 
     /* if we used a gateway on "add route", we also need to specify it on
      * delete, otherwise some OSes will refuse to delete the route