Attempt to fix compiler warning on old compiler

Build farm member lapwing (using gcc 4.7.2) didn't like one part of
9fd45870c1, raising a compiler warning.
Revert that for now.

src/backend/optimizer/path/costsize.c

@@ -2926,12 +2926,13 @@ cost_agg(Path *path, PlannerInfo *root,
 	double		output_tuples;
 	Cost		startup_cost;
 	Cost		total_cost;
-	const AggClauseCosts dummy_aggcosts = {0};
+	AggClauseCosts dummy_aggcosts;
 
 	/* Use all-zero per-aggregate costs if NULL is passed */
 	if (aggcosts == NULL)
 	{
 		Assert(aggstrategy == AGG_HASHED);
+		MemSet(&dummy_aggcosts, 0, sizeof(AggClauseCosts));
 		aggcosts = &dummy_aggcosts;
 	}