I have been in this situation before and I landed the job: Gargle with Salt Water; not honey, not lemon juice.

You might want to chew on a cough drop during the interview, it won't work that great but it is kind of soothing to the person across the table from you. Also, make sure your nose isn't running and that you aren't sniffing. I recommend Zicam chewables.

I promise that you are more worried about it than they are. Make an early joke about it, apologize, but they will forget after a few minutes about your voice if you have a good attitude about it. Ask smart questions and get them talking. My interviewers commented after my interview that they were impressed with my good attitude and that I performed well in the interview DESPITE my lack of voice.