local: speed up multi thread downloads by using sparse files on Windows

Before this change rclone didn't use sparse files on Windows. This
means that when you downloaded a file with multithread download it
wrote the entire file with zeros first on the first write not at the
start of the file.

This change makes the file be sparse on Windows. Linux/macOS files
were already sparse.

backend/local/preallocate_unix.go

@@ -44,3 +44,8 @@ again:
 	// }
 	return err
 }
+
+// setSparse makes the file be a sparse file
+func setSparse(out *os.File) error {
+	return nil
+}