Stop including {brin,gin}_tuple.h in tuplesort.h

Stop including {brin,gin}_tuple.h in tuplesort.h

Doing this meant that those two headers, which are supposed to be
internal to their corresponding index AMs, were being included pretty
much universally, because tuplesort.h is included by execnodes.h which
is very widely used.  Stop that, and fix fallout.

We also change indexing.h to no longer include execnodes.h (tuptable.h
is sufficient), and relscan.h to no longer include buf.h (pointless
since c2fe139c201c).

Author: Mario González <gonzalemario@gmail.com>
Discussion: https://postgr.es/m/CAFsReFUcBFup=Ohv_xd7SNQ=e73TXi8YNEkTsFEE2BW7jS1noQ@mail.gmail.com