Ugh this is tricky.... some girls would want to know and could forgive a drunken kiss (I think I would but have never been in the situation so can't say for certain)... I would definitely rather be told by my boyfriend than hear it off of someone else. It depends on the relationship, I think honesty is so important that if a boyfriend hid this from me it would destroy us.
On the other hand, if something is a one off meaningless kiss that you can barely remember, finding out might not be worth the pain. Different people will have different opinions on this.
It sounds like it was a drunken mistake and a one off - learn from it and make sure it doesn't happen again. It's your call whether you tell her or not, you know your relationship better than anyone here. If your girlfriend would want to know then I think you have to tell her and face the consequences, if you are both right for each other and you explain how sorry you are she might be able to move past it.
If you decide not to tell her, bear in mind that she could find out from somebody else. That could make things a lot worse in the future.