add server time.cloudflare.com.

- Cloudflare have very good adjacency (if PCH did anycast ntp, we'd use it)
- As ntp input, it is great they don't leapsmear
- Not all their nodes do ntp, hope they succeed at scaling that up
- ntpd constrains (un-auth) ntp packets within a TLS constraint window
so there is no downside (unlike pool.ntp entries which slowly decay
but that's a story for another commit..)
ok otto

src/etc/ntpd.conf

@@ -1,7 +1,8 @@
-# $OpenBSD: ntpd.conf,v 1.14 2015/07/15 20:28:37 ajacoutot Exp $
+# $OpenBSD: ntpd.conf,v 1.15 2019/07/04 05:19:31 deraadt Exp $
 #
 # See ntpd.conf(5) and /etc/examples/ntpd.conf
 
 servers pool.ntp.org
+server time.cloudflare.com
 sensor *
 constraints from "https://www.google.com"