By Ryan Vlastelica and Blaise Robinson Mon Dec 31, 2012 3:53am EST Dec 31 (Reuters) – U.S. stock index futures signalled a slightly higher open on Wall Street on Monday, withstocks set to snap a five-session losing streak as talks continued in Washington over resolving the “fiscal cliff”. Futures for the S&P 500 were up 0.43 percent, Dow Jones futures up 0.23…