pg_dump: Add missing relkind case

pg_dump: Add missing relkind case

Checking for RELKIND_MATVIEW was forgotten in
guessConstraintInheritance().  This isn't a live problem, since it is
checked in flagInhTables() which relkinds can have parents, and those
entries will have numParents==0 after that.  But after discussion it
was felt that this place should be kept consistent with
flagInhTables() and flagInhAttrs().

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/a574c8f1-9c84-93ad-a9e5-65233d6fc00f@enterprisedb.com