Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/crosschain/arbitrum/LibArbitrumL1.sol

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+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.8.0) (crosschain/arbitrum/LibArbitrumL1.sol)
+
+pragma solidity ^0.8.4;
+
+import {IBridge as ArbitrumL1_Bridge} from "../../vendor/arbitrum/IBridge.sol";
+import {IOutbox as ArbitrumL1_Outbox} from "../../vendor/arbitrum/IOutbox.sol";
+import "../errors.sol";
+
+/**
+ * @dev Primitives for cross-chain aware contracts for
+ * https://arbitrum.io/[Arbitrum].
+ *
+ * This version should only be used on L1 to process cross-chain messages
+ * originating from L2. For the other side, use {LibArbitrumL2}.
+ */
+library LibArbitrumL1 {
+    /**
+     * @dev Returns whether the current function call is the result of a
+     * cross-chain message relayed by the `bridge`.
+     */
+    function isCrossChain(address bridge) internal view returns (bool) {
+        return msg.sender == bridge;
+    }
+
+    /**
+     * @dev Returns the address of the sender that triggered the current
+     * cross-chain message through the `bridge`.
+     *
+     * NOTE: {isCrossChain} should be checked before trying to recover the
+     * sender, as it will revert with `NotCrossChainCall` if the current
+     * function call is not the result of a cross-chain message.
+     */
+    function crossChainSender(address bridge) internal view returns (address) {
+        if (!isCrossChain(bridge)) revert NotCrossChainCall();
+
+        address sender = ArbitrumL1_Outbox(ArbitrumL1_Bridge(bridge).activeOutbox()).l2ToL1Sender();
+        require(sender != address(0), "LibArbitrumL1: system messages without sender");
+
+        return sender;
+    }
+}