Add virtual to abstract overridden destructor

This is redundant, but required by SWIG.

taglib/fileref.h

@@ -143,7 +143,7 @@ namespace TagLib {
  /*!
  * Destroys this StreamTypeResolver instance.
  */
- ~StreamTypeResolver() override = 0;
+ virtual ~StreamTypeResolver() override = 0; // virtual is needed by SWIG
 
  StreamTypeResolver(const StreamTypeResolver &) = delete;
  StreamTypeResolver &operator=(const StreamTypeResolver &) = delete;