Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/certora/harnesses/ERC20WrapperHarness.sol

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../patched/token/ERC20/extensions/ERC20Wrapper.sol";
+
+contract ERC20WrapperHarness is ERC20Wrapper {
+    constructor(IERC20 _underlying, string memory _name, string memory _symbol) ERC20(_name, _symbol) ERC20Wrapper(_underlying) {}
+
+    function underlyingTotalSupply() public view returns (uint256) {
+        return underlying().totalSupply();
+    }
+
+    function underlyingBalanceOf(address account) public view returns (uint256) {
+        return underlying().balanceOf(account);
+    }
+
+    function underlyingAllowanceToThis(address account) public view returns (uint256) {
+        return underlying().allowance(account, address(this));
+    }
+
+    function recover(address account) public returns (uint256) {
+        return _recover(account);
+    }
+}