aws-iot: Expose ota_agent_task.c statics

This commit makes the static functions in ota_agent_task.c visible
if the UNIT_TESTING macro is defined.
The methodology is documented already in `unit_testing.md`.

This is justified because:
- These functions need to be tested.
- Functions other than `vStartOtaTask` cannot be refactored to be
non-static, as only `vStartOtaTask` should be called in another file.

Signed-off-by: Reuben Cartwright <Reuben.Cartwright@arm.com>