Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/interfaces/IERC2981.sol

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+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (interfaces/IERC2981.sol)
+
+pragma solidity ^0.8.0;
+
+import "../utils/introspection/IERC165.sol";
+
+/**
+ * @dev Interface for the NFT Royalty Standard.
+ *
+ * A standardized way to retrieve royalty payment information for non-fungible tokens (NFTs) to enable universal
+ * support for royalty payments across all NFT marketplaces and ecosystem participants.
+ *
+ * _Available since v4.5._
+ */
+interface IERC2981 is IERC165 {
+    /**
+     * @dev Returns how much royalty is owed and to whom, based on a sale price that may be denominated in any unit of
+     * exchange. The royalty amount is denominated and should be paid in that same unit of exchange.
+     */
+    function royaltyInfo(
+        uint256 tokenId,
+        uint256 salePrice
+    ) external view returns (address receiver, uint256 royaltyAmount);
+}