sensor_combined.msg: make timestamps relative

msg/sensor_combined.msg

...es/attitude_estimator_ekf/attitude_estimator_ekf_main.cpp

...odules/attitude_estimator_q/attitude_estimator_q_main.cpp

src/modules/commander/commander.cpp

src/modules/ekf2/ekf2_main.cpp

src/modules/ekf2_replay/ekf2_replay_main.cpp

...ules/ekf_att_pos_estimator/ekf_att_pos_estimator_main.cpp

src/modules/mavlink/mavlink_messages.cpp

src/modules/mavlink/mavlink_receiver.cpp

.../position_estimator_inav/position_estimator_inav_main.cpp

src/modules/sdlog2/sdlog2.c

src/modules/sensors/sensors.cpp