ddns-scripts: Update knot resolver regexp

ddns-scripts: Update knot resolver regexp

The output format of `khost` has changed. This commit fixes the regexp
for IPv4. It fixes the issue of using a custom DNS to resolve current
address.

```bash
root@localhost:~# khost ns2.afraid.org
ns2.afraid.org. has IPv4 address 69.65.50.223
ns2.afraid.org. has IPv6 address 2001:1850:1:5:800::6b
Host ns2.afraid.org. has no MX record
root@localhost:~# khost --version
khost (Knot DNS), version 3.3.5
```

Signed-off-by: Denis Shulyaka <Shulyaka@gmail.com>