do_length hides overloaded

Got this in a build today (develop):

https://api.travis-ci.org/jobs/292877168/log.txt?deansi=true

```
In file included from libs/program_options/src/convert.cpp:19:
In file included from ./boost/program_options/detail/utf8_codecvt_facet.hpp:19:
./boost/detail/utf8_codecvt_facet.hpp:181:17: warning: 'boost::program_options::detail::utf8_codecvt_facet::do_length' hides overloaded virtual function [-Woverloaded-virtual]
virtual int do_length(
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0/../../../../include/c++/7.2.0/bits/codecvt.h:453:11: note: hidden overloaded virtual function 'std::codecvt::do_length' declared here: type mismatch at 1st parameter ('std::codecvt::state_type &' (aka '__mbstate_t &') vs 'const std::mbstate_t &' (aka 'const __mbstate_t &'))
int do_length(state_type&, const extern_type* __from,
^
```