Fix building with GCC 14
GCC 14 becomes stricter regarging pointer types:
goocanvasitemsimple.c: In function 'goo_canvas_item_simple_set_model':
goocanvasitemsimple.c:1539:15: error: assignment to 'GooCanvasItemModelSimple *' {aka 'struct _GooCanvasItemModelSimple *'} from incompatible pointer type 'GooCanvasItemModel *' {aka 'struct _GooCanvasItemModel *'} [-Wincompatible-pointer-types]
1539 | item->model = g_object_ref (model);
| ^
make[3]: *** [Makefile:595: goocanvasitemsimple.lo] Error 1
It looks like missing a pointer cast.