Fix privileges on pg_statistic_ext.tableoid

Fix privileges on pg_statistic_ext.tableoid

The GRANT in system_views allowed SELECT privileges on various columns in
the pg_statistic_ext catalog, but tableoid was not included in the list.
That made pg_dump fail because it's accessing this column when building
the list of extended statistics to dump.

Discussion: https://postgr.es/m/8833.1560647898%40sss.pgh.pa.us