Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/token/ERC20DecimalsMock.sol

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+import "../../token/ERC20/ERC20.sol";
+
+abstract contract ERC20DecimalsMock is ERC20 {
+    uint8 private immutable _decimals;
+
+    constructor(uint8 decimals_) {
+        _decimals = decimals_;
+    }
+
+    function decimals() public view override returns (uint8) {
+        return _decimals;
+    }
+}