Mark GetDefaultPawnClassForController as BlueprintCallable

We can override GetDefaultPawnClassForController in Blueprint but we can’t call it. When overriding SpawnDefaultPawnFor, it would be nice if we can call GetDefaultPawnClassForController so that if it is overridden, we get its result instead of having to use the GameMode’s DefaultPawnClass variable.

https://github.com/EpicGames/UnrealEngine/pull/4085