Actor Forest Whitaker was at Boston University Monday, where he delivered the fourth Martin Luther King Jr. Leadership Lecture at the school’s Howard Gotlieb Archival Research Center. An Academy Award winner for his performance as Ugandan dictator Idi Amin in “The Last King of Scotland,’’ Whitaker is also well known for his social activism. In 2011, Whitaker was inducted as a UNESCO Goodwill Ambassador for Peace and Reconciliation. The Gotlieb Center is the repository of Whitaker’s personal archive and an exhibition of his material is on display.