Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/utils/Multicall.sol

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (utils/Multicall.sol)
+
+pragma solidity ^0.8.0;
+
+import "./Address.sol";
+
+/**
+ * @dev Provides a function to batch together multiple calls in a single external call.
+ *
+ * _Available since v4.1._
+ */
+abstract contract Multicall {
+    /**
+     * @dev Receives and executes a batch of function calls on this contract.
+     * @custom:oz-upgrades-unsafe-allow-reachable delegatecall
+     */
+    function multicall(bytes[] calldata data) external virtual returns (bytes[] memory results) {
+        results = new bytes[](data.length);
+        for (uint256 i = 0; i < data.length; i++) {
+            results[i] = Address.functionDelegateCall(address(this), data[i]);
+        }
+        return results;
+    }
+}