Fix representation IPv6 DNS servers

The `getNameServerFromResolvd` function ignores the family type of
address received from `systemd-resolvd`. It leads to an incorrect
representation of IPv6 addresses.

This commit fixes that.

Resolves openbmc/phosphor-networkd#34

Testing:
Add an IPv6 DNS server:
```
busctl set-property xyz.openbmc_project.Network \
                    /xyz/openbmc_project/network/eth0 \
                    xyz.openbmc_project.Network.EthernetInterface \
                    StaticNameServers as 1 2001:db8:1::1
```
After the setting will be applied the `Nameservers` property must
contain valid representation:
```
busctl get-property xyz.openbmc_project.Network \
                    /xyz/openbmc_project/network/eth0 \
                    xyz.openbmc_project.Network.EthernetInterface \
                    Nameservers

as 1 "2001:db8:1::1"
```

Change-Id: Id69a0ffcceedd5be28e26ea9b9aca7f37e6a9345
Signed-off-by: Alexander Filippov <a.filippov@yadro.com>
diff --git a/util.hpp b/util.hpp
index 05c400b..7e1aab6 100644
--- a/util.hpp
+++ b/util.hpp
@@ -98,6 +98,8 @@
  * @returns String representation of the ip.
  */
 std::string toString(const InAddrAny& addr);
+std::string toString(const struct in_addr& addr);
+std::string toString(const struct in6_addr& addr);
 
 /* @brief converts the prefix into subnetmask.
  * @param[in] addressFamily - IP address family(AF_INET/AF_INET6).