Use correct preprocessor conditional in relptr.h

Use correct preprocessor conditional in relptr.h

When relptr.h was added (commit fbc1c12a94a), there was no check for
HAVE_TYPEOF, so it used HAVE__BUILTIN_TYPES_COMPATIBLE_P, which
already existed (commit ea473fb2dee) and which was thought to cover
approximately the same compilers.  But the guarded code can also work
without HAVE__BUILTIN_TYPES_COMPATIBLE_P, and we now have a check for
HAVE_TYPEOF (commit 4cb824699e1), so let's fix this up to use the
correct logic.

Co-authored-by: Thomas Munro <[email protected]>
Discussion: https://www.postgresql.org/message-id/CA%2BhUKGL7trhWiJ4qxpksBztMMTWDyPnP1QN%2BLq341V7QL775DA%40mail.gmail.com