Fix MSVC warning needs to have dll-interface (#1185)

Unfortunately, MSVC exports everything (not only public members) when
__declspec(dllexport) is set at the class level via TAGLIB_EXPORT, which
leads to many "needs to have dll-interface to be used by clients" 4251
warnings issued by MSVC, because the std::unique_ptr pimpls are
exported too. As it is not possible to "unexport" private members,
such a warning has to be explicitly suppressed for these cases or
all public and protected class member now have to be exported
for classes derived from a template (StringList, ByteVectorList,
PropertyMap).

taglib/audioproperties.h

@@ -126,6 +126,7 @@ namespace TagLib {
 
   private:
     class AudioPropertiesPrivate;
+    TAGLIB_MSVC_SUPPRESS_WARNING_NEEDS_TO_HAVE_DLL_INTERFACE
     std::unique_ptr<AudioPropertiesPrivate> d;
   };