iOS: Don't send all touch events async

Commit 77942a1bdf introduced the QScopedValueRollback, but without
assigning it to a local temporary, so the value was rolled back
immediately, resulting in always sending touch events async.

Change-Id: Ic7f65c3d38c46813ff06694e883dae3df138b9d4
Reviewed-by: Jake Petroules <jake.petroules@qt.io>

src/plugins/platforms/ios/qiosscreen.mm

@@ -204,7 +204,7 @@ static QIOSScreen* qtPlatformScreenFor(UIScreen *uiScreen)
 
 - (void)sendEvent:(UIEvent *)event
 {
-    QScopedValueRollback<BOOL>(self->_sendingEvent, YES);
+    QScopedValueRollback<BOOL> sendingEvent(self->_sendingEvent, YES);
     [super sendEvent:event];
 }