Broken by previous move to using extern "C" for functions.
This moves the function prototype into the extern "C" block
and gives the functions the correct name when linking.
Review URL: http://webrtc-codereview.appspot.com/333027

git-svn-id: http://libyuv.googlecode.com/svn/trunk@124 16f28f9a-4ce2-e073-06de-1de4eb20be90