Add license headers to vendored Dropbox swagger spec

Author: m90

test/dropbox/user_v2.yaml

@@ -1,3 +1,8 @@
+# Copyright 2018 - Stomakun <https://github.com/stomakun>
+# SPDX-License-Identifier: UNLICENSED
+
+# This file is taken from https://github.com/stomakun/DropboxSwagger
+
 openapi: 3.0.1
 info:
   title: Dropbox User API v2