Include wx/xml/xml.h from the XRC handlers that need it.

Include wx/xml/xml.h from the XRC handlers that need it.

Including this header from wx/xrc/xmlres.h itself only when WXMAKINGDLL_XRC is
defined doesn't work when we're building a static library and not a shared/DLL
one and we don't have any symbol defined in this case, so just don't try to be
smart and include this header from the files that need it.

This fixes breakage of r72756.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72776 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775