Fix switch case fallthrough warning from gcc 11.4.0

[williamleong]

Fixes a warning message from gcc 11.4.0:

.../zmqpp/src/zmqpp/socket.cpp: In member function ‘void zmqpp::socket::set(zmqpp::socket_option, int)’:                                          
.../zmqpp/src/zmqpp/socket.cpp:487:17: warning: this statement may fall through [-Wimplicit-fallthrough=]                                         
  487 |                 if (value < 0) { throw exception("attempting to set a positive only integer option with a negative integer"); }                           
      |                 ^~                                                                                                                                        
...zmqpp/src/zmqpp/socket.cpp:489:9: note: here                                                                                                  
  489 |         case socket_option::reconnect_interval:                                                                                                           
      |         ^~~~