tidy/nit - made TSAN happy on macOS.

Author: talaviram

include/mdns_cpp/mdns.hpp

@@ -46,7 +46,7 @@ class mDNS {
   std::uint16_t port_{42424};
   std::string txt_record_{};
 
-  bool running_{false};
+  std::atomic<bool> running_{false};
   bool dumpMode_{false};
 
   struct sockaddr_in service_address_ipv4_;