nsc: Fix build problem if gtk config store is disabled gtk config store pulled in libdl.so for us, so things fail to link of the config store isn't enabled. This makes nsc pull in libdl itself when its enabled.

#ifndef EMPTY_H
#define EMPTY_H

namespace ns3 {
/**
 * \brief make Callback use a separate empty type
 */
class empty {};
}

#endif /* EMPTY_H */