network: std::unary_function has been deprecated in C++11

There are two occurrences of std::unary_function (i.e. in class Ipv4AddressHash, and Ipv6AddressHash).

➜  ns-3-dev git:(master) ✗ cppgrep unary_function src 
src/network/utils/ipv6-address.h:706:class Ipv6AddressHash : public std::unary_function<Ipv6Address, size_t>
src/network/utils/ipv4-address.h:438:class Ipv4AddressHash : public std::unary_function<Ipv4Address, size_t> {