x11/libxfce4util: fix build on GCC architectures

Add USES=compiler:c11 to fix:
../libxfce4util/xfce-kiosk.h:37: error: redefinition of typedef 'XfceKiosk'
../libxfce4util/xfce-kiosk.h:35: error: previous declaration of 'XfceKiosk' was here

Approved by: mentors (implicit approval)