Fix GCS

Signed-off-by: Olivier Gambier <olivier@docker.com>

vendor/golang.org/x/net/http2/go15.go

@@ -0,0 +1,11 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.5
+
+package http2
+
+import "net/http"
+
+func requestCancel(req *http.Request) <-chan struct{} { return req.Cancel }