gst: avoid unresolved xml symbol
gst: avoid unresolved xml symbol

... for .dll platforms at least, at the expense of a non-operational
gst_xml_write wrapper.

Fixes #398567.

freedesktop-unofficial-mirror/gstreamer-sdk__gst-python,freedesktop-unofficial-mirror/gstreamer-sdk__gst-python,freedesktop-unofficial-mirror/gstreamer-sdk__gst-python,freedesktop-unofficial-mirror/gstreamer-sdk__gst-python