Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/token/ERC1155/extensions/ERC1155Burnable.sol

@@ -0,0 +1,32 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (token/ERC1155/extensions/ERC1155Burnable.sol)
+
+pragma solidity ^0.8.0;
+
+import "../ERC1155.sol";
+
+/**
+ * @dev Extension of {ERC1155} that allows token holders to destroy both their
+ * own tokens and those that they have been approved to use.
+ *
+ * _Available since v3.1._
+ */
+abstract contract ERC1155Burnable is ERC1155 {
+    function burn(address account, uint256 id, uint256 value) public virtual {
+        require(
+            account == _msgSender() || isApprovedForAll(account, _msgSender()),
+            "ERC1155: caller is not token owner or approved"
+        );
+
+        _burn(account, id, value);
+    }
+
+    function burnBatch(address account, uint256[] memory ids, uint256[] memory values) public virtual {
+        require(
+            account == _msgSender() || isApprovedForAll(account, _msgSender()),
+            "ERC1155: caller is not token owner or approved"
+        );
+
+        _burnBatch(account, ids, values);
+    }
+}