databases/pg_citus: fix build with recent postgresql versions

Partially import commit
https://github.com/citusdata/citus/commit/341fdb32fc07a3aea2dda2fa33a70d22819be37b
from upstream (issue https://github.com/citusdata/citus/pull/6711)

PG13 and PG14 have now backported the RelationGetSmgr function that
Citus already backported, so on those PG versions use the function
from the PG sources.

Reported by: pkg-fallout