docker-engine: add iptables runtime dependency

The Docker engine calls iptables internally:

https://github.com/docker/libnetwork/blob/2c921b/iptables/iptables.go#L400

Signed-off-by: Christian Stewart <christian@paral.in>
[Thomas: fix indentation.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/docker-engine/Config.in

@@ -16,6 +16,7 @@ config BR2_PACKAGE_DOCKER_ENGINE_DAEMON
 	depends on BR2_USE_MMU # docker-containerd
 	depends on BR2_USE_WCHAR # docker-containerd
 	select BR2_PACKAGE_DOCKER_CONTAINERD # runtime dependency
+	select BR2_PACKAGE_IPTABLES # runtime dependency
 	select BR2_PACKAGE_SQLITE # runtime dependency
 	default y
 	help