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 16fa9b2b 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

src/backend/access/gist/gistvalidate.c

@@ -338,6 +338,7 @@ gistadjustmembers(Oid opfamilyoid,
 			case GIST_DISTANCE_PROC:
 			case GIST_FETCH_PROC:
 			case GIST_OPTIONS_PROC:
+			case GIST_SORTSUPPORT_PROC:
 				/* Optional, so force it to be a soft family dependency */
 				op->ref_is_hard = false;
 				op->ref_is_family = true;