Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/AccessControlCrossChainMock.sol

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.4;
+
+import "../access/AccessControlCrossChain.sol";
+import "../crosschain/arbitrum/CrossChainEnabledArbitrumL2.sol";
+
+contract AccessControlCrossChainMock is AccessControlCrossChain, CrossChainEnabledArbitrumL2 {}