Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/vendor/arbitrum/IDelayedMessageProvider.sol

@@ -0,0 +1,16 @@
+// Copyright 2021-2022, Offchain Labs, Inc.
+// For license information, see https://github.com/nitro/blob/master/LICENSE
+// SPDX-License-Identifier: BUSL-1.1
+// OpenZeppelin Contracts (last updated v4.8.0) (vendor/arbitrum/IDelayedMessageProvider.sol)
+
+// solhint-disable-next-line compiler-version
+pragma solidity >=0.6.9 <0.9.0;
+
+interface IDelayedMessageProvider {
+    /// @dev event emitted when a inbox message is added to the Bridge's delayed accumulator
+    event InboxMessageDelivered(uint256 indexed messageNum, bytes data);
+
+    /// @dev event emitted when a inbox message is added to the Bridge's delayed accumulator
+    /// same as InboxMessageDelivered but the batch data is available in tx.input
+    event InboxMessageDeliveredFromOrigin(uint256 indexed messageNum);
+}