Try to avoid accidental use of wxXmlNode in XRC handlers.

Try to avoid accidental use of wxXmlNode in XRC handlers.

Now that the handlers can be defined outside of the XRC library, we can't use
wxXmlNode in them because this would create an (unsatisfied) link dependency
on the xml library.

To avoid this happening accidentally, don't include wx/xml/xml.h from the XRC
header, unless building the xrc library itself.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72756 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775