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).
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).