ResolverController - fix NAT64 prefix logging a 'char' instead of an int.

Currently we have:
  D ResolverController: Sending Nat64Prefix added event on netId 100 with address {64:ff9b::(`)}

This is because:
  struct Nat64PrefixInfo {
    unsigned netId;
    bool added;
    std::string prefixString;
    uint8_t prefixLength;
  };

and thus the uint8_t is simply treated as a char(acter).

Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I2a5e1ed7a1d3072c7338b1a1288540682b5514d6