elc_combobox: add/remove item doesn't resize container with genlist

recalculation of item count is getting only on a first load of combobox.
whenever user add more items, size of container with items doesn't getting more
place (provoking scroller to appear).
but its event more annoying when user delete items, then there are lots of
free spaces left that looks ridiculus

@fix

src/lib/elementary/elc_combobox.c

@@ -186,6 +186,8 @@ _activate(Evas_Object *obj)
 
    sd->expanded = EINA_TRUE;
 
+   count_items_genlist(obj);
+
    if (sd->count <= 0) return;
 
    _table_resize(obj);