pgsql: Fix incorrect permissions check in

Log Message:
-----------
Fix incorrect permissions check in information_schema.key_column_usage view:
it was checking a pg_constraint OID instead of pg_class OID, resulting in
"relation with OID nnnnn does not exist" failures for anyone who wasn't
owner of the table being examined. Per bug #2848 from Laurence Rowe.

Note: for existing 8.2 installations a simple version update won't fix this;
the easiest fix is to CREATE OR REPLACE this view with the corrected
definition.

Modified Files:
--------------
pgsql/src/backend/catalog:
information_schema.sql (r1.39 -> r1.40)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/information_schema.sql.diff?r1=1.39&r2=1.40)