We need different export decorators for the different libs.
For now no actual change though, just rename before the release,
and add prelude headers to define the new decorator to GST_EXPORT.
The GstTagXmpWriter interface is to be implemented on elements that
provide xmp serialization. It allows users to select which
xmp schemas should be used on serialization.
API: GstTagXmpWriter
https://bugzilla.gnome.org/show_bug.cgi?id=645167