Can't build on macOS High Sierra with Xcode 9.4.1

The dependency on QTKit fails.

1. There is no header <QTKit/QTKit.h>
2. QTKit was deprecated in macOS 10.9 in favor of AVFoundation: https://developer.apple.com/documentation/qtkit

This prevents building of nui3 on macOS 10.13 with Xcode 9.4.1