Commit 65c5fcd3 broke this by removing a
header include directive that is conditionally required.  Add that back
to nbtree.c, with annotation to keep pgrminclude from re-breaking it.

Peter Geoghegan

Report: <CAM3SWZTNjHFYW_UG8bu0BnogqQ2HfsTgkzXLueuUhfTcYbu5HA@mail.gmail.com>