emotion: gstreamer: fix deprecation warning for g_type_class_add_private

emotion: gstreamer: fix deprecation warning for g_type_class_add_private

Summary:
Since glib 2.58 g_type_class_add_private is deprecated and this throws
an ugly warning in our build. Not being a glib expert I based this on
patch in freeverb which solves the same issue for them:
https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/commit/90625953f27668bde29983e2ad3afe2f49d4133f

Videos in rage are still playing fine for me, but double checking during
review would be appreciated.

Reviewers: raster

Subscribers: cedric, #reviewers, #committers

Tags: #efl

Differential Revision: https://phab.enlightenment.org/D7638