boot: do not build test-hwids-section.c if ukify is disabled

boot: do not build test-hwids-section.c if ukify is disabled

Otherwise the build fails:

```
[22/3075] Generating src/boot/test-hwids-section.c with a custom command (wrapped by meson to capture output)
FAILED: src/boot/test-hwids-section.c
/usr/bin/meson --internal exe --capture src/boot/test-hwids-section.c -- /usr/bin/python3.10 /mnt/work/systemd/upstream-fork/main/src/boot/generate-hwids-section.py /mnt/work/systemd/upstream-fork/main/src/boot/hwids
--- stderr ---
Traceback (most recent call last):
  File "/mnt/work/systemd/upstream-fork/main/src/boot/generate-hwids-section.py", line 12, in <module>
    import ukify
  File "/mnt/work/systemd/upstream-fork/main/src/boot/../ukify/ukify.py", line 61, in <module>
    import pefile  # type: ignore
ModuleNotFoundError: No module named 'pefile'
```

Follow-up for 640f8c3eb8dc21539500e92a5c69cdb6cbda3381