Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/vendor/optimism/ICrossDomainMessenger.sol

@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (vendor/optimism/ICrossDomainMessenger.sol)
+pragma solidity >0.5.0 <0.9.0;
+
+/**
+ * @title ICrossDomainMessenger
+ */
+interface ICrossDomainMessenger {
+    /**********
+     * Events *
+     **********/
+
+    event SentMessage(address indexed target, address sender, bytes message, uint256 messageNonce, uint256 gasLimit);
+    event RelayedMessage(bytes32 indexed msgHash);
+    event FailedRelayedMessage(bytes32 indexed msgHash);
+
+    /*************
+     * Variables *
+     *************/
+
+    function xDomainMessageSender() external view returns (address);
+
+    /********************
+     * Public Functions *
+     ********************/
+
+    /**
+     * Sends a cross domain message to the target messenger.
+     * @param _target Target contract address.
+     * @param _message Message to send to the target.
+     * @param _gasLimit Gas limit for the provided message.
+     */
+    function sendMessage(address _target, bytes calldata _message, uint32 _gasLimit) external;
+}