By Song Sang-ho INCHEON, Jan 13 — Former United Nations Secretary-General Ban Ki-moon on Thursday reiterated his presidential ambitions, calling for joint efforts to bring together a nation suffering from a slew of economic, security and political challenges, South Korea’s Yonhap news agency reported. During the speech he gave after his arrival at Incheon International Airport, west of Seoul, Ban fell short of declaring his presidential bid, but he stressed his resolve to work “for a change of politics, not for a change of government.”