Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/token/ERC721/IERC721Receiver.sol

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.6.0) (token/ERC721/IERC721Receiver.sol)
+
+pragma solidity ^0.8.0;
+
+/**
+ * @title ERC721 token receiver interface
+ * @dev Interface for any contract that wants to support safeTransfers
+ * from ERC721 asset contracts.
+ */
+interface IERC721Receiver {
+    /**
+     * @dev Whenever an {IERC721} `tokenId` token is transferred to this contract via {IERC721-safeTransferFrom}
+     * by `operator` from `from`, this function is called.
+     *
+     * It must return its Solidity selector to confirm the token transfer.
+     * If any other value is returned or the interface is not implemented by the recipient, the transfer will be reverted.
+     *
+     * The selector can be obtained in Solidity with `IERC721Receiver.onERC721Received.selector`.
+     */
+    function onERC721Received(
+        address operator,
+        address from,
+        uint256 tokenId,
+        bytes calldata data
+    ) external returns (bytes4);
+}