Disclose it to them! It's unfortunate that stuttering is always stereotyped with just being anxious when 9 times out of 10, if I'm with a friend who makes me feel comfortable, I end up stuttering but I never want them to take it personally or assume it's because I'm nervous around them when that's not the case whatsoever. It just happens most of the times. Your employer may very much appreciate your honesty of getting it over and done with of "Before we start the interview, if you hear me block up here and there, I have a stutter but it's just a small part of who I am." or something among those lines. Then you move on swiftly and just get on with the questions. They'll get the impression that you're a confident individual regardless and then they won't have any reason to reject you because of a speech impediment as you already told them and it would be discriminatory of them. It will take a lot of pressure off you once it's over and done with!