Fix .gitignore: stop tracking ignored files

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

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../security/PullPayment.sol";
+
+// mock class using PullPayment
+contract PullPaymentMock is PullPayment {
+    constructor() payable {}
+
+    // test helper function to call asyncTransfer
+    function callTransfer(address dest, uint256 amount) public {
+        _asyncTransfer(dest, amount);
+    }
+}