This was previously not covered by allow_system_table_mods, but now it
is.  The impact in practice is probably low, but this makes it
consistent with most other DDL commands.
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/ee9df1af-c0d8-7c82-5be7-39ce4e3b0a9d%402ndquadrant.com