Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/contracts/mocks/proxy/ClashingImplementation.sol

@@ -0,0 +1,17 @@
+// SPDX-License-Identifier: MIT
+
+pragma solidity ^0.8.0;
+
+/**
+ * @dev Implementation contract with a payable admin() function made to clash with TransparentUpgradeableProxy's to
+ * test correct functioning of the Transparent Proxy feature.
+ */
+contract ClashingImplementation {
+    function admin() external payable returns (address) {
+        return 0x0000000000000000000000000000000011111142;
+    }
+
+    function delegatedFunction() external pure returns (bool) {
+        return true;
+    }
+}