add missing include to timestamp.hh
add missing include to timestamp.hh

The file uses std::string but does include <string> header. My compiler
complains.

Message-Id: <830632513ab52bd924820b4d7b95570f81954fc0@scylladb.com>

avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla