Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/interfaces/IERC1967.sol

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (interfaces/IERC1967.sol)
+
+pragma solidity ^0.8.0;
+
+/**
+ * @dev ERC-1967: Proxy Storage Slots. This interface contains the events defined in the ERC.
+ *
+ * _Available since v4.8.3._
+ */
+interface IERC1967 {
+    /**
+     * @dev Emitted when the implementation is upgraded.
+     */
+    event Upgraded(address indexed implementation);
+
+    /**
+     * @dev Emitted when the admin account has changed.
+     */
+    event AdminChanged(address previousAdmin, address newAdmin);
+
+    /**
+     * @dev Emitted when the beacon is changed.
+     */
+    event BeaconUpgraded(address indexed beacon);
+}