Bug 956482 followup - Add license header and fix line endings. r=me

browser/locales/en-US/chrome/browser/aboutPrivateBrowsing.properties

@@ -1,2 +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/.
+
 title=You're browsing privately
 title.normal=Open a private window?