LIBBY GEORGE A group of activists and celebrities, including a co-founder of the Black Lives Matter movement, have signed an open letter to Nigeria's president demanding that he hold accountable security personnel accused of shooting anti-police brutality protesters. The letter, published in the New York Times on Thursday to mark International Human Rights Day, comes nearly two months after what witnesses and Amnesty International say was a fatal clash in Lagos between peaceful protesters and military and police. The military and police deny shooting protesters. The demonstrators had called for an end to police brutality and a much-hated unit called…