Fix missing validation for the new GiST sortsupport functions.

Fix missing validation for the new GiST sortsupport functions.

Because of this, if you tried to create an operator family with the new
sortsupport function, you got an error:

ERROR:  support function number 11 is invalid for access method gist

We missed this in commit 16fa9b2b30 that added the sortsupport function,
because it only added sortsupport to a built-in operator family.

Author: Andrey Borodin
Discussion: https://www.postgresql.org/message-id/3520A18A-5C38-4697-A2E3-F3BDE3496CD5%40yandex-team.ru