mirage/mirage: PR #1515: Make config.ml and unikernel.ml co-buildable

@dc60fae

Stages:

• ✔️ x86_64-debian-10-4.13 (6m47s)
• ✔️ arm64-debian-10-4.13 (5m49s)

History:

• 08/04/2024 14:16 ✔️ @41321bc (9m19s)
• 08/04/2024 14:07 ✔️ @dc60fae (6m47s)