Fix links to callbacks in doxygen.
Fix links to callbacks in doxygen.

HREF based links work in the PDF documentation but not in the HTML
documentation because the callback routines are on a different HTML
page.

Signed-off-by: Ralph Giles <65585f72bf62a11d2f4105aa30c20fe94efea46f@thaumas.net>

xiph/opusfile,xiph/opusfile,xiph/opusfile