emotion: gstreamer: fix deprecation warning for g_type_class_add_private
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:
Videos in rage are still playing fine for me, but double checking during
review would be appreciated.
Differential Revision: https://phab.enlightenment.org/D7638