WASHINGTON (Reuters) - The U.S. Senate overwhelmingly backed a waiver on Thursday that will allow James Mattis to serve as President-elect Donald Trump’s secretary of defense, despite having retired as a Marine General in 2013. The Senate voted 81 to 17 for a one-time waiver of a provision of a law on civilian control of the U.S. military requiring a seven-year wait before active-duty military can lead the Department of Defense. The waiver must still be approved by the House of Representatives Armed Services Committee and full House, and signed into law by the president, to allow Mattis to serve if he is confirmed to lead the Pentagon. 