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/libcharon/plugins/kernel_wfp/kernel_wfp_ipsec.c

@@ -1348,7 +1348,7 @@ static bool manage_route(private_kernel_wfp_ipsec_t *this,
 		return FALSE;
 	}
 	gtw = hydra->kernel_interface->get_nexthop(hydra->kernel_interface,
-											   remote, local);
+											   remote, -1, local);
 	if (add)
 	{
 		done = install_route(this, dst, mask, src, gtw);