fix warning about XfceKiosk redefinition in libxfce4util.h

on openbsd since a while i have this warning building anything that includes libxfce4util.h (so lots of things)

In file included from xfce-shortcut-dialog.c:30:
In file included from /usr/local/include/xfce4/libxfce4util/libxfce4util.h:33:
/usr/local/include/xfce4/libxfce4util/xfce-kiosk.h:37:27: warning: redefinition of typedef 'XfceKiosk' is a C11 feature [-Wtypedef-redefinition]
typedef struct _XfceKiosk XfceKiosk;
                          ^
/usr/local/include/xfce4/libxfce4util/xfce-kiosk.h:35:23: note: previous definition is here
G_DECLARE_FINAL_TYPE (XfceKiosk, xfce_kiosk, XFCE, KIOSK, GObject)
                      ^
1 warning generated.

that's afaict with the default build flags in release mode - commenting line 37 seems to silence the warning and doesnt seem to cause other build issues but that's to check with all libxfce4util consumers..