vendor: update aufs dependency

Signed-off-by: Stephen J Day <stephen.day@docker.com>

vendor/github.com/containerd/aufs/aufs.go

@@ -16,6 +16,7 @@ import (
 
 	"github.com/containerd/containerd/log"
 	"github.com/containerd/containerd/mount"
+	"github.com/containerd/containerd/platforms"
 	"github.com/containerd/containerd/plugin"
 	"github.com/containerd/containerd/snapshots"
 	"github.com/containerd/containerd/snapshots/storage"
@@ -28,6 +29,8 @@ func init() {
 		Type: plugin.SnapshotPlugin,
 		ID:   "aufs",
 		InitFn: func(ic *plugin.InitContext) (interface{}, error) {
+			ic.Meta.Platforms = append(ic.Meta.Platforms, platforms.DefaultSpec())
+			ic.Meta.Exports["root"] = ic.Root
 			return New(ic.Root)
 		},
 	})