Fix NoX11 build

Fix NoX11 build regression not showing Preferences Window. An error
```
-[PreferencesWindowController loadWindow]: failed to load window nib file 'PreferencesWindow'
```
occurs.

The fix is by including NoX11 into the Target Membership of PreferencesWindow.xib. Reference [an answer at StackOverflow](http://stackoverflow.com/a/18291642).