vendor: update btrfs dependency

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

vendor/github.com/containerd/btrfs/helpers.go

@@ -1,3 +1,18 @@
+/*
+  Copyright The containerd Authors
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
 package btrfs
 
 /*