Update vendor

vendor/github.com/docker/engine-api/client/privileged.go

@@ -0,0 +1,9 @@
+package client
+
+// RequestPrivilegeFunc is a function interface that
+// clients can supply to retry operations after
+// getting an authorization error.
+// This function returns the registry authentication
+// header value in base 64 format, or an error
+// if the privilege request fails.
+type RequestPrivilegeFunc func() (string, error)