unix: fix -Wsign-compare warning in tty.c

The mode argument is an enum now and the signedness of an enum is
implementation-defined when it doesn't have negative members.

Cast it to int in the comparison to tty->mode because the latter is
still an int.

PR: https://github.com/libuv/libuv/pull/134
Reviewed-by: Saúl Ibarra Corretgé <saghul@gmail.com>

src/unix/tty.c

@@ -108,7 +108,7 @@ int uv_tty_set_mode(uv_tty_t* tty, uv_tty_mode_t mode) {
   struct termios tmp;
   int fd;
 
-  if (tty->mode == mode)
+  if (tty->mode == (int) mode)
     return 0;
 
   fd = uv__stream_fd(tty);