zerocopy: add SPDX-License-Identifier to source files

[Sagartaunk]

Fixes #3428

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.85%. Comparing base (88393fc) to head (acf3b1b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3432   +/-   ##
=======================================
  Coverage   91.85%   91.85%           
=======================================
  Files          20       20           
  Lines        6093     6093           
=======================================
  Hits         5597     5597           
  Misses        496      496           

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[joshlf]

Tiny nit, but otherwise looks good, thanks! You'll also need to rebase on main.

[joshlf]

Oh and also, you'll need to run ./tools/update-expected-test-output.sh after you've made any changes.

[Sagartaunk]

Oh and also, you'll need to run ./tools/update-expected-test-output.sh after you've made any changes.

Made the changes and rebased on main. Also, are there any good first issues you'd recommend to someone looking to contribute more?