Congratulations on the job offer!
Now, to answer your question, have you already decided to accept the current offer?
YES - then you can attend your interview, and then towards the end, politely communicate to them that you were offered another job. Try to avoid specifying the name of the firm, and your salary - and tell them you'd like to keep the name unspecified if they ask, but tell them about the position/title, and an approx range of your salary. Tell them, that you are thankful to them for giving you an interview, and that you'd be happy to have had an opportunity to work with them, but since time is of essence to you and they haven't concluded the interview process yet, you have made your decision.
NO - meaning, you want to wait for this job results before accepting on the other offer -
Please keep quiet re the other offer, and just go through with this interview process normally. At the end of the interview, you can tell them though, that you would really appreciate if they can let you know about their decision at earliest as they can. If however during the interview they ask you if you've applied at other places, then tell them that yes you have, and that you are waiting for one of them to make an offer (yes, a white lie, its ok considering the survival of the fittest). In such case, also tell them that you'd however be interested in hearing from this firm first, since this opportunity would be your first choice.