fixup! Bug 41916: Letterboxing preferences UI

Import gLetterboxingPrefs for the linter.

browser/components/preferences/main.js

@@ -3,6 +3,7 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 /* import-globals-from extensionControlled.js */
+/* import-globals-from letterboxing.js */
 /* import-globals-from preferences.js */
 /* import-globals-from /toolkit/mozapps/preferences/fontbuilder.js */
 /* import-globals-from /browser/base/content/aboutDialog-appUpdater.js */