fix build

[eschmidbauer]

fixes build error

whisper-cpp-server/handler/inference_handler.cpp:447:37: error: unknown type name 'queue_mutex'
  std::unique_lock<std::mutex> lock(queue_mutex);
                                    ^
whisper-cpp-server/handler/inference_handler.cpp:447:36: warning: parentheses were disambiguated as a function declaration [-Wvexing-parse]
  std::unique_lock<std::mutex> lock(queue_mutex);
                                   ^~~~~~~~~~~~~
whisper-cpp-server/handler/inference_handler.cpp:447:37: note: add a pair of parentheses to declare a variable
  std::unique_lock<std::mutex> lock(queue_mutex);