New port: audio/gstreamer1-plugins-sndio

The GStreamer sndio plugin allows applications that use GStreamer to
play and record to/from an sndio server.

This includes WebKit-based browsers, video players like Totem, and
audio players like Rhythmbox.

WWW: https://github.com/t6/gstreamer1-plugins-sndio/

PR: 215904
Approved by: lme (mentor)
Differential Revision: https://reviews.freebsd.org/D9622