contracts-bedrock: add gov token to predeploys contract

The `GovernanceToken` never had a field in the `Predeploys`
contract, this commit adds it.

packages/contracts-bedrock/contracts/libraries/Predeploys.sol

@@ -104,4 +104,9 @@ library Predeploys {
      * @notice Address of the L1FeeVault predeploy.
      */
     address internal constant L1_FEE_VAULT = 0x420000000000000000000000000000000000001A;
+
+    /**
+     * @notice Address of the GovernanceToken predeploy.
+     */
+    address internal constant GOVERNANCE_TOKEN = 0x4200000000000000000000000000000000000042;
 }