libs: make sure gobject-introspection scanner calls gst_init()

libs: make sure gobject-introspection scanner calls gst_init()

Fixes introspection failures caused by type assertions/warnings.
Since we now moved from _get_type() functions to external GType
variables in a couple of places, we actually have to call gst_init()
to make sure these are set when we use GST_TYPE_FOO.