Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/certora/harnesses/ERC721Harness.sol

@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../patched/token/ERC721/ERC721.sol";
+
+contract ERC721Harness is ERC721 {
+    constructor(string memory name, string memory symbol) ERC721(name, symbol) {}
+
+    function mint(address account, uint256 tokenId) external {
+        _mint(account, tokenId);
+    }
+
+    function safeMint(address to, uint256 tokenId) external {
+        _safeMint(to, tokenId);
+    }
+
+    function safeMint(address to, uint256 tokenId, bytes memory data) external {
+        _safeMint(to, tokenId, data);
+    }
+
+    function burn(uint256 tokenId) external {
+        _burn(tokenId);
+    }
+
+    function tokenExists(uint256 tokenId) external view returns (bool) {
+        return _exists(tokenId);
+    }
+
+    function unsafeOwnerOf(uint256 tokenId) external view returns (address) {
+        return _ownerOf(tokenId);
+    }
+
+    function unsafeGetApproved(uint256 tokenId) external view returns (address) {
+        return _getApproved(tokenId);
+    }
+}