gst/gstobject.h: Don't define xmlNodePtr to gpointer if the core was built with

gst/gstobject.h: Don't define xmlNodePtr to gpointer if the core was built with

Original commit message from CVS:
Patch by: Peter Kjellerstedt <pkj at axis com>
* gst/gstobject.h:
Don't define xmlNodePtr to gpointer if the core was built with
--disable-loadsave and --disable-registry, this will break
applications that want to use libxml2 but are buildling against a
core that doesn't use libxml2. Use an intermediary type GstXmlNodePtr
instead so we don't have to mess with the libxml2 namespace
(#361675).