Reorder two nbtree.h function prototypes.

Make the function prototype order consistent with the definition order
in nbtinsert.c.

src/include/access/nbtree.h

@@ -730,8 +730,8 @@ extern void _bt_parallel_advance_array_keys(IndexScanDesc scan);
  */
 extern bool _bt_doinsert(Relation rel, IndexTuple itup,
 						 IndexUniqueCheck checkUnique, Relation heapRel);
-extern Buffer _bt_getstackbuf(Relation rel, BTStack stack, BlockNumber child);
 extern void _bt_finish_split(Relation rel, Buffer bbuf, BTStack stack);
+extern Buffer _bt_getstackbuf(Relation rel, BTStack stack, BlockNumber child);
 
 /*
  * prototypes for functions in nbtsplitloc.c