database.cc: Fix compilation error with boost 1.55
database.cc: Fix compilation error with boost 1.55

Message-Id: <70f62aadb69be73d1c580a9832c60bb727992b49@scylladb.com>

kjniemi/scylla,duarten/scylla,raphaelsc/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,kjniemi/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla,kjniemi/scylla,raphaelsc/scylla,avikivity/scylla