kernel-net: Let get_nexthop() return an optional interface name

The returned name should be the interface over which the destination
address/net is reachable.

src/libcharon/kernel/kernel_interface.c

@@ -524,13 +524,14 @@ METHOD(kernel_interface_t, get_source_addr, host_t*,
 }
 
 METHOD(kernel_interface_t, get_nexthop, host_t*,
-	private_kernel_interface_t *this, host_t *dest, int prefix, host_t *src)
+	private_kernel_interface_t *this, host_t *dest, int prefix, host_t *src,
+	char **iface)
 {
 	if (!this->net)
 	{
 		return NULL;
 	}
-	return this->net->get_nexthop(this->net, dest, prefix, src);
+	return this->net->get_nexthop(this->net, dest, prefix, src, iface);
 }
 
 METHOD(kernel_interface_t, get_interface, bool,