This fixes following build failure with Boost 1.67.0:
```
In file included from /usr/include/boost/optional/optional.hpp:33,
from /usr/include/boost/optional.hpp:15,
from ./thin-provisioning/emitter.h:23,
from contrib/thin_sexp_emitter.cc:2:
/usr/include/boost/optional/optional.hpp: In instantiation of ‘std::basic_ostream<_CharT, _Traits>& boost::operator<<(std::basic_ostream<_CharT, _Traits>&, const boost::optional_detail::optional_tag&) [with CharType = char; CharTrait = std::char_traits<char>]’:
./base/indented_stream.h:31:9: required from ‘{anonymous}::indented_stream& {anonymous}::indented_stream::operator<<(const T&) [with T = boost::optional<unsigned int>]’
contrib/thin_sexp_emitter.cc:105:29: required from ‘void {anonymous}::sexp_emitter::kv(const char*, const T&) [with T = boost::optional<unsigned int>]’
contrib/thin_sexp_emitter.cc:29:21: required from here
/usr/include/boost/optional/optional.hpp:1481:3: error: static assertion failed: If you want to output boost::optional, include header <boost/optional/optional_io.hpp>
BOOST_STATIC_ASSERT_MSG(sizeof(CharType) == 0, "If you want to output boost::optional, include header <boost/optional/optional_io.hpp>");
^~~~~~~~~~~~~~~~~~~~~~~
make: *** [contrib/Makefile:15: contrib/thin_sexp_emitter.o] Error 1
```
boost/optional/optional_io.hpp exists since at least Boost 1.34.0, so it is
safe to include in older versions, too.
