remove typedef already defined by G_DECLARE_FINAL_TYPE macro (fixes #4)

building with clang yields this warning on OpenBSD:

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)

i'm no gobject expert and G_DECLARE_FINAL_TYPE doesnt seem much used in other Xfce areas but that typedef has been here since cae460a7 16 years ago...