Make wxXmlResourceHandler::IsOfClass() static.

Make wxXmlResourceHandler::IsOfClass() static.

This simple helper function doesn't use any wxXmlResourceHandler data as it's
just a trivial wrapper for wxXmlNode::GetAttribute().

Making it static allows, in particular, to call it from const member functions
of wxXmlResourceHandler-derived classes (making it "const" itself would
achieve this too, of course, but it just doesn't need to be non-static).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65888 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775