Remove unnecessary (and wrong) forward declaration.
Interestingly only C++ compilers have, so far, complained about this
odd forward declaration. This originated when IndexBuildCallback was
defined in another file, but now is completely unnecessary (but was
wrong before too, cpluspluscheck just wouldn't have noticed).
Reported-By: Tom Lane
Discussion: https://postgr.es/m/53941.
1559239260@sss.pgh.pa.us