Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/TimersBlockNumberImpl.sol

@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../utils/Timers.sol";
+
+contract TimersBlockNumberImpl {
+    using Timers for Timers.BlockNumber;
+
+    Timers.BlockNumber private _timer;
+
+    function getDeadline() public view returns (uint64) {
+        return _timer.getDeadline();
+    }
+
+    function setDeadline(uint64 timestamp) public {
+        _timer.setDeadline(timestamp);
+    }
+
+    function reset() public {
+        _timer.reset();
+    }
+
+    function isUnset() public view returns (bool) {
+        return _timer.isUnset();
+    }
+
+    function isStarted() public view returns (bool) {
+        return _timer.isStarted();
+    }
+
+    function isPending() public view returns (bool) {
+        return _timer.isPending();
+    }
+
+    function isExpired() public view returns (bool) {
+        return _timer.isExpired();
+    }
+}