dtls_debug.c: fix output of IP addresses for Contiki

* UIP_CONF_IPV6 is now called NETSTACK_CONF_WITH_IPV6
* As sprintf() is used anyway when NDEBUG is not set, we can also
  use it for printing IPv4 addresses.

Change-Id: I91247ba0292c23a2e389a4a6a8f15c6204473b5f
1 file changed