include/denc: include used header

include/denc: include used header

to address the FTBFS like:

/home/kchai/ceph/src/include/denc.h: In member function ‘void DencDumper<T>::dump() const’:
/home/kchai/ceph/src/include/denc.h:121:60: error: ‘O_BINARY’ was not declared in this scope
     int fd = ::open(fn, O_WRONLY|O_TRUNC|O_CREAT|O_CLOEXEC|O_BINARY, 0644);
                                                            ^~~~~~~~
/home/kchai/ceph/src/include/denc.h:121:60: note: the macro ‘O_BINARY’ had not yet been defined
In file included from /home/kchai/ceph/src/include/statlite.h:14,
                 from /home/kchai/ceph/src/include/types.h:41,
                 from /home/kchai/ceph/src/auth/Crypto.h:19,
                 from /home/kchai/ceph/src/auth/Crypto.cc:21:

Signed-off-by: Kefu Chai <kchai@redhat.com>