dropboxQt::files::ListFolderResult missing virtual destructor

[barbicels]

Re: https://github.com/osoftteam/dropboxQt/blob/master/src/dropbox/files/FilesListFolderResult.h

This class has virtual methods, but is missing the virtual destructor that all other dropboxQt::files class headers have.
Because of this, clang posts a warning wherever an object of this class is deleted.

It just needs this line added:
virtual ~ListFolderResult(){}