GUANTANAMO BAY U.S. NAVAL BASE, Cuba, March 26 (Reuters) - Australian prisoner David Hicks agreed on Monday to plead guilty before a U.S. military tribunal to a charge of providing material support to terrorists. The 31-year-old former kangaroo skinner was accused of fighting for al Qaeda in Afghanistan during the U.S.-led invasion in 2001 and has been held at Guantanamo for more than five years. He announced his decision in a hearing in the U.S. war crimes tribunal at the base.