Fix -Wdeprecated-copy

[georgthegreat]

boost/circular_buffer/include/boost/circular_buffer/details.hpp:263:15: error: definition of implicit copy constructor for 'iterator<boost::circular_buffer<bool>, boost::cb_details::const_traits<std::allocator<bool>>>' is deprecated because it has a user-declared copy assignment operator [-Werror,-Wdeprecated-copy]
    iterator& operator=(const iterator&) = default;
              ^

[georgthegreat]

I think that compiler-generated assignment operator should work as intended.

[CamJN]

Just commenting in the hopes this gets looked at.