x11/xfce4-verve-plugin: fix build on GCC architectures

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

from /usr/local/include/xfce4/libxfce4ui-2/libxfce4ui/libxfce4ui.h:23,
from verve.c:34:

/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)