Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/token/ERC777Mock.sol

@@ -0,0 +1,13 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../../token/ERC777/ERC777.sol";
+
+abstract contract ERC777Mock is ERC777 {
+    event BeforeTokenTransfer();
+
+    function _beforeTokenTransfer(address, address, address, uint256) internal override {
+        emit BeforeTokenTransfer();
+    }
+}