Fix building wxMSW GDI+ graphics code in non-Unicode build

Use wc_str() with GDI+ function which always takes wide strings, even in
non-Unicode build.

Closes #18172.

src/msw/graphics.cpp

@@ -2335,7 +2335,7 @@ void wxGDIPlusRenderer::Load()
             for ( size_t i = 0 ; i < n; i++ )
             {
                 const wxString& fname = privateFonts[i];
-                gs_privateFonts->AddFontFile(fname.t_str());
+                gs_privateFonts->AddFontFile(fname.wc_str());
             }
 
             gs_pFontFamily = new Gdiplus::FontFamily[n];