devel/boost-libs: backport minor regression fix

/usr/local/include/boost/lockfree/spsc_queue.hpp:140:43: error: no member named 'next' in namespace 'boost'

const ConstIterator last = boost::next(begin, input_count);
                           ~~~~~~~^

/usr/local/include/boost/lockfree/spsc_queue.hpp:145:51: error: no member named 'next' in namespace 'boost'

const ConstIterator midpoint = boost::next(begin, count0);
                               ~~~~~~~^

Obtained from: upstream (via Homebrew)