kernel-interface: Add destination prefix to get_nexthop()

This allows to determine the next hop to reach a subnet, for instance, when
installing routes for shunt policies.

src/libhydra/plugins/kernel_netlink/kernel_netlink_net.c

@@ -1818,7 +1818,7 @@ METHOD(kernel_net_t, get_source_addr, host_t*,
 }
 
 METHOD(kernel_net_t, get_nexthop, host_t*,
-	private_kernel_netlink_net_t *this, host_t *dest, host_t *src)
+	private_kernel_netlink_net_t *this, host_t *dest, int prefix, host_t *src)
 {
 	return get_route(this, dest, TRUE, src, 0);
 }