examples/externalsocket.c: fix SunPro compilation issue

#define IPADDR "127.0.0.1"

#define PORTNUM 80

#ifndef INADDR_NONE

#define INADDR_NONE 0xffffffff

#endif

static size_t write_data(void *ptr, size_t size, size_t nmemb, void *stream)

{

  int written = fwrite(ptr, size, nmemb, (FILE *)stream);