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/dsdiff/dsdiffdiintag.h

@@ -137,6 +137,7 @@ namespace TagLib {
 
       private:
         class TagPrivate;
+        TAGLIB_MSVC_SUPPRESS_WARNING_NEEDS_TO_HAVE_DLL_INTERFACE
         std::unique_ptr<TagPrivate> d;
       };
     }  // namespace DIIN

taglib/dsdiff/dsdifffile.h

@@ -283,6 +283,7 @@ namespace TagLib {
                       unsigned int leadingPadding = 0);
 
       class FilePrivate;
+      TAGLIB_MSVC_SUPPRESS_WARNING_NEEDS_TO_HAVE_DLL_INTERFACE
       std::unique_ptr<FilePrivate> d;
     };
   }  // namespace DSDIFF

taglib/dsdiff/dsdiffproperties.h

@@ -70,6 +70,7 @@ namespace TagLib {
 
     private:
       class PropertiesPrivate;
+      TAGLIB_MSVC_SUPPRESS_WARNING_NEEDS_TO_HAVE_DLL_INTERFACE
       std::unique_ptr<PropertiesPrivate> d;
     };
   }  // namespace DSDIFF