Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/certora/harnesses/InitializableHarness.sol

@@ -0,0 +1,23 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.2;
+
+import "../patched/proxy/utils/Initializable.sol";
+
+contract InitializableHarness is Initializable {
+    function initialize()          public initializer      {}
+    function reinitialize(uint8 n) public reinitializer(n) {}
+    function disable()             public { _disableInitializers(); }
+
+    function nested_init_init()                     public initializer      { initialize();    }
+    function nested_init_reinit(uint8 m)            public initializer      { reinitialize(m); }
+    function nested_reinit_init(uint8 n)            public reinitializer(n) { initialize();    }
+    function nested_reinit_reinit(uint8 n, uint8 m) public reinitializer(n) { reinitialize(m); }
+
+    function version() public view returns (uint8) {
+        return _getInitializedVersion();
+    }
+
+    function initializing() public view returns (bool) {
+        return _isInitializing();
+    }
+}