Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/token/ERC1155/utils/ERC1155Holder.sol

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.5.0) (token/ERC1155/utils/ERC1155Holder.sol)
+
+pragma solidity ^0.8.0;
+
+import "./ERC1155Receiver.sol";
+
+/**
+ * Simple implementation of `ERC1155Receiver` that will allow a contract to hold ERC1155 tokens.
+ *
+ * IMPORTANT: When inheriting this contract, you must include a way to use the received tokens, otherwise they will be
+ * stuck.
+ *
+ * @dev _Available since v3.1._
+ */
+contract ERC1155Holder is ERC1155Receiver {
+    function onERC1155Received(
+        address,
+        address,
+        uint256,
+        uint256,
+        bytes memory
+    ) public virtual override returns (bytes4) {
+        return this.onERC1155Received.selector;
+    }
+
+    function onERC1155BatchReceived(
+        address,
+        address,
+        uint256[] memory,
+        uint256[] memory,
+        bytes memory
+    ) public virtual override returns (bytes4) {
+        return this.onERC1155BatchReceived.selector;
+    }
+}