Suppress compiler warnings in non-cassert builds.

With Asserts off, these variables are set but never used, resulting
in warnings from pickier compilers.  Fix that with our standard solution.
Per report from Jeff Janes.

src/backend/access/gist/gistutil.c

@@ -852,7 +852,7 @@ gistproperty(Oid index_oid, int attno,
 			 bool *res, bool *isnull)
 {
 	HeapTuple	tuple;
-	Form_pg_index rd_index;
+	Form_pg_index rd_index PG_USED_FOR_ASSERTS_ONLY;
 	Form_pg_opclass rd_opclass;
 	Datum		datum;
 	bool		disnull;

src/backend/utils/adt/amutils.c

@@ -115,7 +115,7 @@ test_indoption(Oid relid, int attno, bool guard,
 			   bool *res)
 {
 	HeapTuple	tuple;
-	Form_pg_index rd_index;
+	Form_pg_index rd_index PG_USED_FOR_ASSERTS_ONLY;
 	Datum		datum;
 	bool		isnull;
 	int2vector *indoption;