[WFLY-1341] fixed the fix (just a typo)

Here's a fix for the original fix (just a typo). There's no such package as `sun.nio.ext`, it should have been `sun.nio.cs.ext` from the very beginning. The mistake is described in https://bugzilla.redhat.com/show_bug.cgi?id=959478 and if you look at the customer case linked from the BZ, you can see that the BZ was originally filed wrongly.