Gruntfile.js needs to be ignored

[andre68723]

running with ui5-migration migrate in a ui folder containing a Gruntfile.js causes issues since that is wrapped inside sap.ui

git diff
+++ b/ui-app/Gruntfile.js
@@ -1,3 +1,5 @@
+sap.ui.define([], function() {
+	  "use strict";
 	module.exports = function(grunt) {
 	  "use strict";
 	  grunt.initConfig({
@@ -21,3 +23,4 @@
 	    "build"
 	  ]);
 	};
+});
`
``