rust: zerocopy: add SPDX License Identifiers

rust: zerocopy: add SPDX License Identifiers

Originally, when the Rust upstream `alloc` standard library crate was
vendored, the SPDX License Identifiers were added to every file so that
the license on those was clear. The same happened with the vendoring of
`proc_macro2`, `quote` and `syn`. Please see:

  commit 057b8d257107 ("rust: adapt `alloc` crate to the kernel")
  commit 69942c0a8965 ("rust: syn: add SPDX License Identifiers")
  commit ddfa1b279d08 ("rust: quote: add SPDX License Identifiers")
  commit a9acfceb9614 ("rust: proc-macro2: add SPDX License Identifiers")

Thus do the same for the `zerocopy` crate.

This makes `scripts/spdxcheck.py` pass: use parentheses like commit
06e9bfc1e57d ("ionic: make spdxcheck.py happy") did since we have two
`OR` operators in the expression (three licenses).

SPDX identifiers are not added to the `benches` files because they are
included in rendered documentation. Nevertheless, the `README.md` to be
added by a later commit mentions the license.

Finally, as requested, I filed an issue [1] with upstream about it.

Cc: Joshua Liebow-Feeser <joshlf@google.com>
Cc: Jack Wrenn <jswrenn@google.com>
Link: https://github.com/google/zerocopy/issues/3428
Link: https://patch.msgid.link/20260608141439.182634-10-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>