Replace IpPort::is_global with IpAddr::is_global from std

[kpp]

tox/src/toxcore/dht/ip_port.rs

Line 7 in 76840d9

/// TODO: replace with https://doc.rust-lang.org/std/net/struct.Ipv4Addr.html#method.is_global when it is stabilized

See rust-lang/rust#60145
Still we need it to be in stable