Add MPL 2.0 license header to multiple CSS and SVG files

src/toolkit/themes/shared/icons/indicator-private-browsing.svg

@@ -1 +1,6 @@
+<!--
+   - 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 http://mozilla.org/MPL/2.0/.
+   -->
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