gst_writev: define UIO_MAXIOV on iOS/OSX
gst_writev: define UIO_MAXIOV on iOS/OSX

Apparently it's only seton iOS/OSX if defined(KERNEL).

centricular/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,centricular/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,krichter722/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,centricular/gstreamer,jpxiong/gstreamer,centricular/gstreamer