x11-fm/thunar: fix build on GCC architectures

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

from /usr/local/include/exo-2/exo/exo-enum-types.h:6,
from /usr/local/include/exo-2/exo/exo.h:39,
from ../thunar/thunar-enum-types.h:24,
from ../thunar/thunar-file.h:28,
from ../thunar/thunar-chooser-button.h:23,
from thunar-chooser-button.c:27:

/usr/local/include/xfce4/libxfce4util/xfce-kiosk.h:37: error: redefinition of typedef 'XfceKiosk'

Approved by: mentors (implicit approval)