(Reuters) - A former lawmaker filed a petition at Kenya s Supreme Court on Monday challenging the results of last month s presidential election, a Reuters witness said, opening the door to potential legal challenges to President Uhuru Kenyatta s re-election.  The petition, filed by former legislator Harun Mwau, seeks to overturn Kenyatta s victory in the Oct. 26 election, which was held after the Supreme Court nullified Kenyatta s win in an Aug. 8 presidential election on procedural grounds. The grounds for Mwau s petition were not immediately clear since his lawyer did not answer questions.  