x11-wm/xfce4-session: fix build on GCC architectures

Use C11 compiler:
/usr/local/include/xfce4/libxfce4util/xfce-kiosk.h: At top level:
/usr/local/include/xfce4/libxfce4util/xfce-kiosk.h:37: error: redefinition of typedef 'XfceKiosk'
/usr/local/include/xfce4/libxfce4util/xfce-kiosk.h:35: error: previous declaration of 'XfceKiosk' was here

Approved by: mentors (implicit approval)