Mayor Bloomberg was snubbed by one of his younger constituents at yesterday’s groundbreaking for a police precinct house on Staten Island. After the press conference, Bloomberg attempted to give a high-five to a 7-year-old girl — but was rebuffed. “She left him hanging,” laughs an eyewit ness. “A bunch of the photog raphers who saw it were all laughing.” The precinct house, in Graniteville, is slated to cost $63.6 million.