Add missing license header to Makefile

[progval]

This fixes the CI license checks.

@WenyXu is the sole author of this file

[progval]

ugh, but not all license checks:

error[rejected]: failed to satisfy license requirements
  ┌─ zerovec-derive 0.10.3 (registry+https://github.com/rust-lang/crates.io-index):4:12
  │
4 │ license = "Unicode-3.0"
  │            ^^^^^^^^^^^
  │            │
  │            license expression retrieved via Cargo.toml `license`
  │            rejected: license was not explicitly allowed

[waynexia]

ugh, but not all license checks:

This looks like another issue, I think we can merge this now to fix the header one