ipxedust: add nap.h to arm64 build to set NAP_NULL to support u-boot

[rpardini]

ipxedust: add nap.h to arm64 build to set NAP_NULL to support u-boot

• see https://github.com/ipxe/ipxe/discussions/605#discussioncomment-2273515
• "This will cause iPXE to leave the CPU spinning instead of putting it to sleep, and will avoid the problems on U-Boot."
• Seems harmless on non U-Boot machines -- the CPU will spin during the key timeout (2s?) but that's about it.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.82%. Comparing base (52f7e0f) to head (fc5ed25). Report is 7 commits behind head on main.

:exclamation: Current head fc5ed25 differs from pull request most recent head 3db501c. Consider uploading reports for the commit 3db501c to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #152 +/- ## ======================================= Coverage 97.82% 97.82% ======================================= Files 5 5 Lines 367 367 ======================================= Hits 359 359 Misses 4 4 Partials 4 4 ```

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