Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/utils/introspection/ERC165.sol

@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.1 (utils/introspection/ERC165.sol)
+
+pragma solidity ^0.8.0;
+
+import "./IERC165.sol";
+
+/**
+ * @dev Implementation of the {IERC165} interface.
+ *
+ * Contracts that want to implement ERC165 should inherit from this contract and override {supportsInterface} to check
+ * for the additional interface id that will be supported. For example:
+ *
+ * ```solidity
+ * function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
+ *     return interfaceId == type(MyInterface).interfaceId || super.supportsInterface(interfaceId);
+ * }
+ * ```
+ *
+ * Alternatively, {ERC165Storage} provides an easier to use but more expensive implementation.
+ */
+abstract contract ERC165 is IERC165 {
+    /**
+     * @dev See {IERC165-supportsInterface}.
+     */
+    function supportsInterface(bytes4 interfaceId) public view virtual override returns (bool) {
+        return interfaceId == type(IERC165).interfaceId;
+    }
+}