include/buffer: include <memory>

include/buffer: include <memory>

to address following FTBFS:

/usr/bin/ccache /usr/bin/clang++-13 -DBOOST_ALL_NO_LIB -DBOOST_ASIO_DISABLE_CONCEPTS -DBOOST_ASIO_DISABLE_THREAD_KEYWORD_EXTENSION -DBOOST_ASIO_USE_TS_EXECUTOR_AS_DEFAULT -DBOOST_PROGRAM_OPTIONS_DYN_LINK -DBOOST_T$
In file included from /var/ssd/ceph/src/crimson/os/seastore/seastore_types.cc:4:
In file included from /var/ssd/ceph/src/crimson/os/seastore/seastore_types.h:14:
In file included from /var/ssd/ceph/src/include/denc.h:47:
/var/ssd/ceph/src/include/buffer.h:98:37: error: no template named 'unique_ptr' in namespace 'std'; did you mean 'boost::movelib::unique_ptr'?
struct unique_leakable_ptr : public std::unique_ptr<T, ceph::nop_delete<T>> {
                                    ^~~~~~~~~~~~~~~
                                    boost::movelib::unique_ptr
/opt/ceph/include/boost/move/unique_ptr.hpp:354:7: note: 'boost::movelib::unique_ptr' declared here
class unique_ptr
      ^

Signed-off-by: Kefu Chai <tchaikov@gmail.com>