Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/interfaces/IERC1271.sol

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+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.1 (interfaces/IERC1271.sol)
+
+pragma solidity ^0.8.0;
+
+/**
+ * @dev Interface of the ERC1271 standard signature validation method for
+ * contracts as defined in https://eips.ethereum.org/EIPS/eip-1271[ERC-1271].
+ *
+ * _Available since v4.1._
+ */
+interface IERC1271 {
+    /**
+     * @dev Should return whether the signature provided is valid for the provided data
+     * @param hash      Hash of the data to be signed
+     * @param signature Signature byte array associated with _data
+     */
+    function isValidSignature(bytes32 hash, bytes memory signature) external view returns (bytes4 magicValue);
+}