Fix license headers for less files

Summary: Failing OSS requirements due to missing headers right now: P143086435

Reviewed By: mweststrate

Differential Revision: D23843532

fbshipit-source-id: 759daefbec641d59adff6ecee8d06dd90c7a6b5b

desktop/themes/base.less

@@ -1,3 +1,10 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ */
 @import '../node_modules/antd/dist/antd.less';
 /* Based on: https://www.figma.com/file/4e6BMdm2SuZ1L7FSuOPQVC/Flipper?node-id=620%3A84636 */
 @border-radius-base: 6px;

desktop/themes/dark.less

@@ -1,3 +1,10 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ */
 @import '../node_modules/antd/lib/style/themes/dark.less';
 @import './base.less';
 

desktop/themes/light.less

@@ -1,3 +1,10 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ */
 @import '../node_modules/antd/lib/style/themes/default.less';
 @import './base.less';