Correct type of front_pathkey to PathKey

In sort_inner_and_outer we iterate a list of PathKey elements, but the
variable is declared as (List *). This mistake is benign, because we
only pass the pointer to lcons() and never dereference it.

This exists since ~2004, but it's confusing. So fix and backpatch to all
supported branches.

Backpatch-through: 10
Discussion: https://postgr.es/m/bf3a6ea1-a7d8-7211-0669-189d5c169374%40enterprisedb.com

src/backend/optimizer/path/joinpath.c

@@ -1258,7 +1258,7 @@ sort_inner_and_outer(PlannerInfo *root,
 
 	foreach(l, all_pathkeys)
 	{
-		List	   *front_pathkey = (List *) lfirst(l);
+		PathKey	   *front_pathkey = (PathKey *) lfirst(l);
 		List	   *cur_mergeclauses;
 		List	   *outerkeys;
 		List	   *innerkeys;