MINOR: Rat should ignore generated directories (#7729)

For some reason, PR builds are failing due to the `rat` license
check even though it should ignore files included in `.gitignore`.

Reviewers: Jason Gustafson <jason@confluent.io>

build.gradle

@@ -122,7 +122,8 @@ if (new File('.git').exists()) {
         '**/id_rsa',
         '**/id_rsa.pub',
         'checkstyle/suppressions.xml',
-        'streams/quickstart/java/src/test/resources/projects/basic/goal.txt'
+        'streams/quickstart/java/src/test/resources/projects/basic/goal.txt',
+        '**/generated/**'
     ])
   }
 }