Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/token/ERC1155/extensions/IERC1155MetadataURI.sol

@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.1 (token/ERC1155/extensions/IERC1155MetadataURI.sol)
+
+pragma solidity ^0.8.0;
+
+import "../IERC1155.sol";
+
+/**
+ * @dev Interface of the optional ERC1155MetadataExtension interface, as defined
+ * in the https://eips.ethereum.org/EIPS/eip-1155#metadata-extensions[EIP].
+ *
+ * _Available since v3.1._
+ */
+interface IERC1155MetadataURI is IERC1155 {
+    /**
+     * @dev Returns the URI for token type `id`.
+     *
+     * If the `\{id\}` substring is present in the URI, it must be replaced by
+     * clients with the actual token type ID.
+     */
+    function uri(uint256 id) external view returns (string memory);
+}