DumperRawBanks.cpp: avoid anonymous class instances

gcc 14 warns if we do not give name to instances of classes:

../Dumpers/BinaryDumpers/src/DumpRawBanks.cpp: In member function 'virtual void DumpRawBanks::operator()(const std::array<std::tuple<std::vector<char, std::allocator<char> >, int>, 102>&, const LHCb::ODIN&) const':
../Dumpers/BinaryDumpers/src/DumpRawBanks.cpp:80:32: warning: ignoring return value of 'std::lock_guard<_Mutex>::lock_guard(mutex_type&) [with _Mutex = std::mutex; mutex_type = std::mutex]', declared with attribute 'nodiscard' [-Wunused-result]
   80 |     std::lock_guard {m_dirMutex};
      |                                ^