By THÉRÈSE MARGOLIS Mexican pop star Gloria Trevi is slated to headline the Freixenet vendimia (grape harvest festival) in the northern central Mexican state of Querétaro on Friday, Aug. 2. Trevi, often referred to as the “Mexican Madonna,” will be performing during the first night of the three-day vendimia. The festival, which is organized each year at Freixemet Mexico’s Finca