`handle_message_hil_gps` incorrectly populates `time_utc_usec` field

[obicons]

Describe the bug

Inside of the message handler for handle_message_hil_gps, the time_utc_usec field is written to with the value of the message's time_usec field. This is incorrect because MAVLink says that the timestamp format is permitted to be in time since system boot.

To Reproduce

Steps to reproduce the behavior:

1. Run drone in HITL
2. Send message with time_usec in a different format

Expected behavior

The timestamp should be converted to the correct format.