xdg-su: Fix escaping for lxqt-sudo

Closed Fabio Pesari requested to merge fpesari/xdg-utils:master into master

With the current escaping (double quotes) lxqt-sudo won't launch programs with more than one argument. With this fix, it will be able to do it. Fixes nasty bugs such as this.

I release this small change under the same license (MIT) as the xdg-utils project.

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