Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/utils/introspection/IERC1820Implementer.sol

@@ -0,0 +1,20 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.1 (utils/introspection/IERC1820Implementer.sol)
+
+pragma solidity ^0.8.0;
+
+/**
+ * @dev Interface for an ERC1820 implementer, as defined in the
+ * https://eips.ethereum.org/EIPS/eip-1820#interface-implementation-erc1820implementerinterface[EIP].
+ * Used by contracts that will be registered as implementers in the
+ * {IERC1820Registry}.
+ */
+interface IERC1820Implementer {
+    /**
+     * @dev Returns a special value (`ERC1820_ACCEPT_MAGIC`) if this contract
+     * implements `interfaceHash` for `account`.
+     *
+     * See {IERC1820Registry-setInterfaceImplementer}.
+     */
+    function canImplementInterfaceForAddress(bytes32 interfaceHash, address account) external view returns (bytes32);
+}