Make WinMtfFontStyle's base class EMFIO_DLLPUBLIC, too

...otherwise the dynamic_cast<WinMtfFontStyle*> in MtfTools::SelectObject
(emfio/source/reader/mtftools.cxx) would not work on macOS in
CppunitTest_emfio_wmf

Change-Id: I5c361544fd273bdb65a4b83c7d694bae714815f5

emfio/inc/mtftools.hxx

@@ -295,7 +295,7 @@ namespace emfio
         void        AddPolyPolygon(const tools::PolyPolygon& rPolyPolygon);
     };
 
-    struct GDIObj
+    struct EMFIO_DLLPUBLIC GDIObj
     {
         virtual ~GDIObj() = default; // Polymorphic base class
     };