crypto/xform.c: migrate to SPDX identifier

Most tools used for compliance and SBOM generation use SPDX identifiers
This change brings us a step closer to an easy SBOM generation.

NOTE
The code was reported as GPL by FOSS ID
and Xiaomi scanned the file xform.c with Black Duck Security and it showed
that the license was BSD-3-Clause and no risk was reported.

Since there is no clause on the license it was concluded as 0BSD

Refference
apache#15252

Signed-off-by: Alin Jerpelea <[email protected]>

crypto/xform.c

@@ -1,6 +1,18 @@
 /****************************************************************************
  * crypto/xform.c
- * $OpenBSD: xform.c,v 1.61 2021/10/22 12:30:53 bluhm Exp $
+ *
+ * SPDX-License-Identifier: 0BSD
+ * SPDX-FileCopyrightText: 1995, 1996, 1997, 1998, 1999 John Ioannidis
+ * SPDX-FileCopyrightText: 1995, 1996, 1997, 1998, 1999 Angelos D. Keromytis
+ * SPDX-FileCopyrightText: 1995, 1996, 1997, 1998, 1999 Niels Provos.
+ * SPDX-FileCopyrightText: 2001 Angelos D. Keromytis.
+ * SPDX-FileCopyrightText: 2008 Damien Miller
+ * SPDX-FileCopyrightText: 2010, 2015 Mike Belopuhov
+ * SPDX-FileContributor: John Ioannidis ([email protected])
+ * SPDX-FileContributor: Angelos D. Keromytis ([email protected])
+ * SPDX-FileContributor: Niels Provos ([email protected])
+ * SPDX-FileContributor: Damien Miller ([email protected])
+ * SPDX-FileContributor: Mike Belopuhov ([email protected])
  *
  * The authors of this code are John Ioannidis ([email protected]),
  * Angelos D. Keromytis ([email protected]),