uefi-raw: make padding more explicit

[phip1611]

Make the padding more explicit.

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)

[nicholasbishop]

We used to have that padding field, but took it out in #629

I think not having the padding field is fine, as this is a repr(C) struct and the C definition does not include an explicit padding field:
https://uefi.org/specs/UEFI/2.10/07_Services_Boot_Services.html#efi-boot-services-getmemorymap

Is there a particular reason why this struct should get explicit padding? (I'm sure there are lots of other structs in UEFI that have implicit padding too.)

[phip1611]

Not a specific reason. Just for better readability.

Consistency is more important, however. Either we should have explicit padding everywhere or not at all.