Fix incorrect permissions check in information_schema.key_column_usage view:

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.