Export public wxDelegateTheme class.

This class is supposed to be public but wasn't accessible in shared library
build as it didn't have public visibility.

Closes #11051.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61570 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/univ/theme.h

@@ -83,7 +83,7 @@ private:
 // will be left to the original theme
 // ----------------------------------------------------------------------------
 
-class wxDelegateTheme : public wxTheme
+class WXDLLIMPEXP_CORE wxDelegateTheme : public wxTheme
 {
 public:
     wxDelegateTheme(const wxString& theme);