U.S. retail sales fell in December as unseasonably warm weather undercut purchases of winter apparel and cheaper gasoline weighed on receipts at service stations, the latest indication that economic growth braked sharply in the fourth quarter. Other data on Friday showed producer prices fell last month on weak energy costs and modest gains in the price of services, suggesting that inflation could struggle to reach the Federal Reserve’s 2 percent target this year. The Commerce Department said retail sales slipped 0.1 percent after rising 0.4 percent in November.