downloadbuffer: drop unneeded macros for G_OS_WIN32
downloadbuffer: drop unneeded macros for G_OS_WIN32

StreamUtils/gstreamer,StreamUtils/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,jpxiong/gstreamer,centricular/gstreamer,centricular/gstreamer,jpxiong/gstreamer