Clang/str_list.h: fix clang warning about std::move

Clang/str_list.h: fix clang warning about std::move

/home/jenkins/workspace/ceph-master/src/include/str_list.h:99:10: warning: moving a local object in a return statement prevents copy elision [-Wpessimizing-move]
  return std::move(str_vec);
         ^
/home/jenkins/workspace/ceph-master/src/include/str_list.h:99:10: note: remove std::move call here
  return std::move(str_vec);
         ^~~~~~~~~~       ~
1 warning generated.

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>