add SPDX header to integration contracts

Signed-off-by: xermicus <cyrill@parity.io>

crates/integration/contracts/ERC20.sol

@@ -1,4 +1,5 @@
 // SPDX-License-Identifier: MIT
+
 pragma solidity ^0.8.20;
 
 // https://github.com/OpenZeppelin/openzeppelin-contracts/blob/v3.0.0/contracts/token/ERC20/IERC20.sol
@@ -9,7 +10,10 @@ interface IERC20 {
 
     function transfer(address recipient, uint amount) external returns (bool);
 
-    function allowance(address owner, address spender) external view returns (uint);
+    function allowance(
+        address owner,
+        address spender
+    ) external view returns (uint);
 
     function approve(address spender, uint amount) external returns (bool);
 
@@ -23,9 +27,6 @@ interface IERC20 {
     event Approval(address indexed owner, address indexed spender, uint value);
 }
 
-
-
-
 contract ERC20 is IERC20 {
     uint public totalSupply;
     mapping(address => uint) public balanceOf;