The federal annex building on Spring Street now officially is the Martin Luther King Jr. federal building. President Reagan signed a law renaming the building Thursday. It makes the annex the first federal building in the nation to bear King's name. ``I think it's appropriate that the honor comes to Atlanta because this is the birthplace of Dr. King,'' said Rep. John Lewis, who introduced the bill in December. The federal government began a $10 million renovation project three years ago at the 1930s building. Agencies that have moved in since include the Food and Nutrition Service, the Immigration and Naturalization Service and the Air Force Engineering Office.