message: fix set_log_file issue
diff: #include "multicast_streamer.h"
  #include "stub_streamer.h"
  +#include "spdlog/sinks/null_sink.h"
  +
  #define LOGGER_NAME "brainflow_logger"
  @@ -30,10 +32,13 @@ int Board::set_log_level (int level)
  int Board::set_log_file (char *log_file)
  {
  spdlog::level::level_enum level = Board::board_logger->level ();
  + Board::board_logger = spdlog::create<spdlog::sinks::null_sink_st> (
  + "null_logger"); // to dont set logger to nullptr and avoid raice condition
  spdlog::drop (LOGGER_NAME);
  Board::board_logger = spdlog::basic_logger_mt (LOGGER_NAME, log_file);
  Board::board_logger->set_level (level);
  Board::board_logger->flush_on (level);
  + spdlog::drop ("null_logger");
  return STATUS_OK;
  }
  