remove MSVC warnings about DLL-exported class inheriting from non DLL-exported one by not exporting private classes at all

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@59481 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/graphics.cpp

@@ -357,7 +357,7 @@ private:
     DECLARE_DYNAMIC_CLASS_NO_COPY(wxGDIPlusContext)
 };
 
-class WXDLLIMPEXP_CORE wxGDIPlusMeasuringContext : public wxGDIPlusContext
+class wxGDIPlusMeasuringContext : public wxGDIPlusContext
 {
 public:
     wxGDIPlusMeasuringContext( wxGraphicsRenderer* renderer ) : wxGDIPlusContext( renderer , m_hdc = GetDC(NULL), 1000, 1000 )