I'm with Tamara; just doing the movement is enough of a stretch that you'll increase mobility over time. Especially if you focus on getting as low as you can in every rep.
That being said, everybody's body is different, so some people's knees will slide forward more than others. I don't think it's something that you should actively be trying for. Just load up the bar, squat, and consciously get as deep as you can fighting for an upright back.