examples: add missing include

Patch fixes following build error:

error: implicit declaration of function ‘close’ [-Werror=implicit-function-declaration]
  close(cl->s.fd.fd);

Signed-off-by: Luka Perkov <luka@openwrt.org>

examples/ustream-example.c

@@ -5,6 +5,7 @@
 #include <getopt.h>
 #include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 
 #include "ustream.h"
 #include "uloop.h"