Add Q_UNUSED for parameter (OS X)

Ifdefs exclude the code on OS X, but there is still unused parameter
and annoying compiler (either warning or error, depending on ...).

Change-Id: I913c25b4f96ebe275a6b1d15873bccf4aebe9e7a
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>

src/widgets/widgets/qtabbar.cpp

@@ -2036,6 +2036,8 @@ void QTabBar::wheelEvent(QWheelEvent *event)
     int offset = event->delta() > 0 ? -1 : 1;
     d->setCurrentNextEnabledIndex(offset);
     QWidget::wheelEvent(event);
+#else
+    Q_UNUSED(event)
 #endif
 }
 #endif //QT_NO_WHEELEVENT