Fix inconsistent parameter names between prototype and declaration

Noticed while working in this area.  This code was introduced in PG15,
which is still in beta, so backpatch to there for consistency.

Backpatch-through: 15

src/include/optimizer/paths.h

@@ -208,7 +208,7 @@ extern int	group_keys_reorder_by_pathkeys(List *pathkeys,
 										   List **group_clauses);
 extern List *get_useful_group_keys_orderings(PlannerInfo *root, double nrows,
 											 List *path_pathkeys,
-											 List *pathkeys, List *clauses);
+											 List *group_pathkeys, List *group_clauses);
 extern Path *get_cheapest_path_for_pathkeys(List *paths, List *pathkeys,
 											Relids required_outer,
 											CostSelector cost_criterion,