Billionaire investor William Ackman’s Pershing Square Holdings hedge fund ended 2015 with a 20.5 percent loss after Valeant Pharmaceuticals, a top holding, has battered in the second half of the year. The fund, one of several portfolios run by the firm, ended December nearly flat with a 0.3 percent gain for the month, Pershing Square, one of the industry’s most closely watched hedge funds, told investors in an update. News of the sharp drop wasn’t a surprise since Ackman told investors two weeks ago that the year would likely end up being the worst in the firm’s 11 year history.