[openssl/openssl] cda3c1: Add missing copyright header

[Pauli]

  Branch: refs/heads/openssl-3.1
  Home:   https://github.com/openssl/openssl
  Commit: cda3c1d92acb3fdb28637d98ac85aef761350995
      https://github.com/openssl/openssl/commit/cda3c1d92acb3fdb28637d98ac85aef761350995
  Author: Pauli <pauli at openssl.org>
  Date:   2023-03-15 (Wed, 15 Mar 2023)

  Changed paths:
    M crypto/bn/rsa_sup_mul.c

  Log Message:
  -----------
  Add missing copyright header

This file was only recently introduced and the missing header slipped through
the review process.

Fixes #20461

Reviewed-by: Richard Levitte <levitte at openssl.org>
Reviewed-by: Tomas Mraz <tomas at openssl.org>
(Merged from https://github.com/openssl/openssl/pull/20470)

(cherry picked from commit 93b0a1ea614f9ce3931373fd3d1d1af04795e6d7)