build: Add missing libxfce4util flags

This will be necessary when xfwm4 is built against a meson release of libxfce4ui, which sets private flags by default in pkgconfig (and therefore no longer exposes libxfce4util flags).

approved this merge request

added 2 commits

• 4535cd34 - 1 commit from branch xfce:master
• 6468f80e - build: Add missing libxfce4util flags

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merged