sql: add missing obj_description case

Found with the following query:

```sql
COMMENT ON SCHEMA public IS 'hello';

SELECT obj_description(oid, 'pg_namespace')
FROM pg_namespace WHERE nspname = 'public';
```

Release note (sql change): The PostgreSQL compatibility
function `obj_description` now supports retrieving
comments on schemas.

pkg/sql/sem/builtins/pg_builtins.go

@@ -2169,6 +2169,8 @@ func getCatalogOidForComments(catalogName string) (id int, ok bool) {
 		return catconstants.PgCatalogDescriptionTableID, true
 	case "pg_constraint":
 		return catconstants.PgCatalogConstraintTableID, true
+	case "pg_namespace":
+		return catconstants.PgCatalogNamespaceTableID, true
 	default:
 		// We currently only support comments on pg_class objects
 		// (columns, tables) in this context.