I once had the same situation and solved it by putting the question up for grabs. First answer at all would get marked as ‘correct answer’. Just to get it marked as answered, added a comment explaining this.
It's currently not possible to mark your own answer as the accepted answer, possibly to avoid abuse with gathering points. I agree that it would be a nice feature though, although I hope they would make sure you don't get experience points for marking your own answer.
It might be nice to add this as an idea to the ideas forum: