contracts-bedrock: adds spdx header to weth98 (#9921)

Without the header, it results in a warning message.
This prevents CI from passing.
We also want to ensure that its free software.

packages/contracts-bedrock/src/dispute/weth/WETH98.sol

+// SPDX-License-Identifier: GPL-3.0
 // Copyright (C) 2015, 2016, 2017 Dapphub
 
 // This program is free software: you can redistribute it and/or modify