Fix a compiler warning when -Wshadow is used.

Fix a compiler warning when -Wshadow is used.

    CC       serialport.lo
  In file included from ../serialport.c:25:0:
  ../serialport.c: In function 'get_config':
  ../libserialport_internal.h:227:25: warning: declaration of 'ret' shadows a previous local [-Wshadow]
   #define TRY(x) do { int ret = x; if (ret != SP_OK) RETURN_CODEVAL(ret); } while (0)
                           ^
  ../serialport.c:1566:3: note: in expansion of macro 'TRY'
     TRY(get_baudrate(port->fd, &config->baudrate));
     ^
  ../serialport.c:1543:6: warning: shadowed declaration is here [-Wshadow]
    int ret = get_flow(port->fd, data);
        ^

Also, add -Wshadow to the list of default compiler options.