settings: Fix untranslatable string

The new option "Cycle through minimized windows in most recently used
order" wasn't marked as translatable.

As result, translators had no way to translate it in transiflex.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>
Fixes: 841fcf27 - Add `cycle_minimized` option
Closes: #527
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