Stock futures flat as investors brace for economic data

* After a roller-coaster ride sparked by Britain's June 23 vote to leave the European Union, investors in the United States are looking for signs of the health of the economy and the possible impact of the vote on quarterly earnings. * Investors will keenly watch for the June payrolls report on Friday, especially after a shockingly weak May report threw the Fed off track from its plans to raise interest rates in the near term. * U.S. economic data in the past weeks, including manufacturing and s