Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/token/ERC721/extensions/ERC721Burnable.sol

@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.8.0) (token/ERC721/extensions/ERC721Burnable.sol)
+
+pragma solidity ^0.8.0;
+
+import "../ERC721.sol";
+import "../../../utils/Context.sol";
+
+/**
+ * @title ERC721 Burnable Token
+ * @dev ERC721 Token that can be burned (destroyed).
+ */
+abstract contract ERC721Burnable is Context, ERC721 {
+    /**
+     * @dev Burns `tokenId`. See {ERC721-_burn}.
+     *
+     * Requirements:
+     *
+     * - The caller must own `tokenId` or be an approved operator.
+     */
+    function burn(uint256 tokenId) public virtual {
+        //solhint-disable-next-line max-line-length
+        require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: caller is not token owner or approved");
+        _burn(tokenId);
+    }
+}