Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/interfaces/IERC6372.sol

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts (last updated v4.9.0) (interfaces/IERC6372.sol)
+
+pragma solidity ^0.8.0;
+
+interface IERC6372 {
+    /**
+     * @dev Clock used for flagging checkpoints. Can be overridden to implement timestamp based checkpoints (and voting).
+     */
+    function clock() external view returns (uint48);
+
+    /**
+     * @dev Description of the clock
+     */
+    // solhint-disable-next-line func-name-mixedcase
+    function CLOCK_MODE() external view returns (string memory);
+}