Fix .gitignore: stop tracking ignored files

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

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../patched/utils/structs/EnumerableSet.sol";
+
+contract EnumerableSetHarness {
+    using EnumerableSet for EnumerableSet.Bytes32Set;
+
+    EnumerableSet.Bytes32Set private _set;
+
+    function add(bytes32 value) public returns (bool) {
+        return _set.add(value);
+    }
+
+    function remove(bytes32 value) public returns (bool) {
+        return _set.remove(value);
+    }
+
+    function contains(bytes32 value) public view returns (bool) {
+        return _set.contains(value);
+    }
+
+    function length() public view returns (uint256) {
+        return _set.length();
+    }
+
+    function at_(uint256 index) public view returns (bytes32) {
+        return _set.at(index);
+    }
+
+    function _indexOf(bytes32 value) public view returns (uint256) {
+        return _set._inner._indexes[value];
+    }
+}