Wings, I have it on a vinyl bootleg called COLD CUTS, but I was still just as surprised as you when I heard it at the beginning of that movie too. And of all his unreleased stuff, why that one? Only Paul knows why it's not on the soundtrack, but I'd guess that it probably has something to do with money.
Paul has tons of good unreleased stuff that will hopefully see the light of day in some kind of anthology of rarities. Sometimes the odd tune makes it onto a CD single as a B-cut.