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.

Tags:
----
REL8_2_STABLE

Modified Files:
--------------
    pgsql/src/backend/catalog:
        information_schema.sql (r1.38 -> r1.38.2.1)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/information_schema.sql.diff?r1=1.38&r2=1.38.2.1)