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.