mavlink: set correct distance_sensor timestamp. Fixes #11840

When PX4FLOW is connected to PX4 through MAVLink (e.g., through a USB
port), the timestamp assigned to the distance_sensor was wrong. This fix
uses the same timestamp assigned to the optical_flow message created from
the same OPTICAL_FLOW_RAD MAVLink message.

Signed-off-by: Gabriel Moreno <gabrielm@cs.cmu.edu>