Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/token/ERC4626OffsetMock.sol

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../../token/ERC20/extensions/ERC4626.sol";
+
+abstract contract ERC4626OffsetMock is ERC4626 {
+    uint8 private immutable _offset;
+
+    constructor(uint8 offset_) {
+        _offset = offset_;
+    }
+
+    function _decimalsOffset() internal view virtual override returns (uint8) {
+        return _offset;
+    }
+}