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

Use C11 compiler:
In file included from /usr/local/include/xfce4/libxfce4util/libxfce4util.h:33,

from xfdesktop-common.c:51:

/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)