Unfortunately for Williamson, this mix-up of attributions will probably last forever. It's all over the Internet attributed to Mandela, as in
Most of the time it is claimed to have been in his "inaugural speech, 1994."
He made at least two inauguration speeches that year, and neither of them contain anything even close to it. Some say it is in one of his "lesser known speeches," but no verification of that seems to be possible, and even if it is, he would have been quoting Williamson and apparently not giving her credit for it.
Mandela's inauguration speeches, 1994:
Union Building, Pretoria 10 May 1994
http://www.polity.org.za/html/govdocs/s ... bookmark=1
Grand Parade, Cape Town, 9 May 1994
http://www.polity.org.za/html/govdocs/s ... 0509a.html