Export recently added wxRichTextXMLHelper to fix link errors.

Export recently added wxRichTextXMLHelper to fix link errors.

This class is referenced from outside the library, see e.g. this build log:

http://buildbot.tt-solutions.com/wx/builders/Linux x86 wxGTK trunk/builds/2961/steps/compile utils/logs/stdio

and so must be exported, otherwise the build fails under Linux (and under
Windows when using DLL).

Perhaps it would be better to make it completely private to the library if it
is not meant to be public, but this would require more changes.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74763 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775