common/ceph_time: move operator<<(ostream&, timespan&) into std namespace

common/ceph_time: move operator<<(ostream&, timespan&) into std namespace

otherwise compiler is not able to find it as the "timespan" here is
actually a class defined in std namespace, even it has an alias defined
in ceph namespace like:

typedef std::chrono::duration<rep, std::nano> timespan;

but this does not make it a member of "ceph" namespace. for more details
on the lookup rules, see https://en.cppreference.com/w/cpp/language/adl

Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 75aafcba888a5753d2a4a8378637b4bb9fad5dd0)