Get rid of some Q_WS_X11.

This macro isn't defined in Qt5 anymore, so use Q_OS_* where
appropriate.

Regarding bug QTCREATORBUG-7936:
Now the fonts look normal again on GNU/Linux, e.g. in the text editor
and the application/compile output pane.

Task-number: QTCREATORBUG-7936
Change-Id: I501cca608f273789d095e1138b32c2c8c38b0059
Reviewed-by: Eike Ziller <eike.ziller@digia.com>

src/plugins/debugger/stackwindow.cpp

@@ -225,9 +225,7 @@ void StackTreeView::copyContentsToClipboard()
         str += QLatin1Char('\n');
     }
     QClipboard *clipboard = QApplication::clipboard();
-#    ifdef Q_WS_X11
     clipboard->setText(str, QClipboard::Selection);
-#    endif
     clipboard->setText(str, QClipboard::Clipboard);
 }