Hide "static constexpr auto" from SWIG

SWIG can't parse "static constexpr auto" at present. As there's no need
to SWIG private class members simply hiding the expression is all that's
needed.

include/exiv2/datasets.hpp

@@ -332,8 +332,9 @@ namespace Exiv2 {
         IptcKey* clone_() const override;
 
         // DATA
+#ifndef SWIG
         static constexpr auto familyName_ = "Iptc";
-
+#endif
         uint16_t tag_;                 //!< Tag value
         uint16_t record_;              //!< Record value
         std::string key_;              //!< Key