Fix C23 compiler warning

The approach of declaring a function pointer with an empty argument
list and hoping that the compiler will not complain about casting it
to another type no longer works with C23, because foo() is now
equivalent to foo(void).

We don't need to do this here.  With a few struct forward declarations
we can supply a correct argument list without having to pull in
another header file.

(This is the only new warning with C23.  Together with the previous
fix a67a49648d, this makes the whole code compile cleanly under C23.)

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/95c6a9bf-d306-43d8-b880-664ef08f2944%40eisentraut.org

src/include/nodes/pathnodes.h

@@ -1107,6 +1107,9 @@ typedef struct IndexOptInfo IndexOptInfo;
 #define HAVE_INDEXOPTINFO_TYPEDEF 1
 #endif
 
+struct IndexPath;				/* avoid including pathnodes.h here */
+struct PlannerInfo;				/* avoid including pathnodes.h here */
+
 struct IndexOptInfo
 {
 	pg_node_attr(no_copy_equal, no_read, no_query_jumble)
@@ -1206,7 +1209,7 @@ struct IndexOptInfo
 	bool		amcanmarkpos;
 	/* AM's cost estimator */
 	/* Rather than include amapi.h here, we declare amcostestimate like this */
-	void		(*amcostestimate) () pg_node_attr(read_write_ignore);
+	void		(*amcostestimate) (struct PlannerInfo *, struct IndexPath *, double, Cost *, Cost *, Selectivity *, double *, double *) pg_node_attr(read_write_ignore);
 };
 
 /*