Fix support for PostgreSQL-12 ("getrelid" macro removed)

Fix support for PostgreSQL-12 ("getrelid" macro removed)

The call is replaced with its expansion which should work
as back as PostgreSQL-9.1 which is before the oldest version
PostGIS-2.2 (our oldest maintained branch) supports.

This is then safe to backport to all branches till 2.2.

Patch by Laurenz Albe

References #4306 in 2.5 branch

git-svn-id: http://svn.osgeo.org/postgis/branches/2.5@16918 b70326c6-7e19-0410-871a-916f4a2858ee