msg: fix includes for ostringstream

msg: fix includes for ostringstream

Clang complains...

```
/home/jenkins/workspace/ceph-master/src/msg/msg_types.h:420:19: error: implicit instantiation of undefined template 'std::__1::basic_ostringstream<char, std::__1::char_traits<char>, std::__1::allocator<char> >'
    ostringstream ss;
                  ^
/usr/include/c++/v1/iosfwd:123:32: note: template is declared here
    class _LIBCPP_TEMPLATE_VIS basic_ostringstream;
                               ^
1 error generated.
```

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>