Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/proxy/beacon/IBeacon.sol

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.1 (proxy/beacon/IBeacon.sol)
+
+pragma solidity ^0.8.0;
+
+/**
+ * @dev This is the interface that {BeaconProxy} expects of its beacon.
+ */
+interface IBeacon {
+    /**
+     * @dev Must return an address that can be used as a delegate call target.
+     *
+     * {BeaconProxy} will check that this address is a contract.
+     */
+    function implementation() external view returns (address);
+}