The same thing they always get. You aren't forcing them to lay, not technically. Just helping them. I think changing their food just b/c of season change might not be good for them. Mine get layer crumbles and scratch grain, all the time.
The only thing I would do is add some high energy treats. More calories, protein, and carbs in the treats. I use sunflower seeds and the occasional block of suet. Their regular feed I would keep the same.
Ours are free range and we don't believe in artificial light to encourage laying either. We do give ours Aviacharge in the wintertime. We haven't fed them layer this winter but will start giving them layer as spring approaches, end of February or March, probably.
We like to give them a little supplement in the winter time to supplement the protein and other things they miss out on when they can't free range, and we think it helps keep them healthier overall. Last winter we gave them kelp meal instead of vitamins, it works well too and is cheaper but you have to be sparing with it, it has a lot of salt in it. We feed cracked corn and scratch and feed twice to three times a day in winter, evening feeding is right before they go to roost.