New port: audio/gstreamer-plugins-sndio

The GStreamer sndio plugin allows applications that use GStreamer 0.10
to play and record through sndio.

This includes several games, audio players like Exaile, and audio
editors like Jokosher.

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

Approved by: lme (mentor)
Differential Revision: https://reviews.freebsd.org/D10656