Silence warning about sign comparison on some Linux distributions

../../ntpd/ntp_io.c:3295:7: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

We know that cnt is non-negative at this point and it should be safe to cast to
unsigned.

https://bugs.webkit.org/show_bug.cgi?id=102061

 took a similar approach in the
WebKit codebase.

Signed-off-by: Gary E. Miller <gem@rellim.com>