Is it the pedal linkage? If so, use some silicon spray sparingly on it... do NOT spray into the opening going into the booster.
If that does not fix it, it may be the master cylinder seals... try flushing your brake fluid. The fresh brake fluid will have fresh additives that reduce the friction and should stop the squeak. (use a turkey baster to empty the reservoir first, then fill it up and do some pedal bleeds to get the fresh fluid into the system.)