Add missing licence text (#286)

src/screens/util/Extensions.ts

@@ -1,3 +1,10 @@
+/*
+ * Copyright (C) Contributors to the Suwayomi project
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
 import { IExtension, TranslationKey } from 'typings';
 import { DefaultLanguage, langCodeToName } from 'util/language';
 import { t } from 'i18next';