copyright header for legacy platform

src/platform/web/LegacyPlatform.js

@@ -1,3 +1,19 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+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.
+*/
+
 import aesjs from "../../../lib/aes-js/index.js";
 import {hkdf} from "../../utils/crypto/hkdf.js";
 import {Platform as ModernPlatform} from "./Platform.js";