Add missing headers

Author: jtpio

packages/tree-extension/style/base.css

@@ -1,3 +1,9 @@
+/*-----------------------------------------------------------------------------
+| Copyright (c) Jupyter Development Team.
+|
+| Distributed under the terms of the Modified BSD License.
+|----------------------------------------------------------------------------*/
+
 .jp-FileBrowser-toolbar .jp-Toolbar-item.jp-DropdownMenu,
 .jp-FileBrowser-toolbar .jp-Toolbar-item.jp-ToolbarButton,
 .jp-FileBrowser-toolbar .jp-Toolbar-item.jp-CommandToolbarButton

packages/tree-extension/style/index.css

@@ -1,5 +1,11 @@
+/*-----------------------------------------------------------------------------
+| Copyright (c) Jupyter Development Team.
+|
+| Distributed under the terms of the Modified BSD License.
+|----------------------------------------------------------------------------*/
+
 @import url('~@jupyterlab/filebrowser/style/index.css');
 
 @import url('~@jupyter-notebook/tree/style/index.css');
 
-@import url('./base.css');
+@import './base.css';