Cops say a driver fatally struck a man who may have been sleeping in a cardboard box under the Gowanus Expressway in Sunset Park last weekend. Officers with Brooklyn’s 72nd Precinct responded to an emergency call about a pedestrian hit by a car on Friday, July 9 at around 10:21 pm on Third Avenue between 52nd and 53rd streets, according to authorities. When they arrived, cops say they found the victim — a 46-year-old man — lying in the…