Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/test/helpers/create2.js

@@ -0,0 +1,11 @@
+function computeCreate2Address(saltHex, bytecode, deployer) {
+  return web3.utils.toChecksumAddress(
+    `0x${web3.utils
+      .sha3(`0x${['ff', deployer, saltHex, web3.utils.soliditySha3(bytecode)].map(x => x.replace(/0x/, '')).join('')}`)
+      .slice(-40)}`,
+  );
+}
+
+module.exports = {
+  computeCreate2Address,
+};