Whistling up some luck?

In “Thirteen Days,” Kevin Costner drives Bobby Kennedy to the crucial meeting with the Soviet rep. When they get there, he tells Bobby that he’ll wait outside for him, and try to “whistle up some luck.” While Bobby’s in the meeting, he can hear Costner whistling “Danny Boy.”

I’d never heard this expression before. Does anyone know anything about it? (It’s not in Brewer’s.)