lib/crc32: add SPDX license identifier

lib/crc32.c and include/linux/crc32.h got missed by the bulk SPDX
conversion because of the nonstandard explanation of the license.
However, crc32.c clearly states that it's licensed under the GNU General
Public License, Version 2.  And the comment in crc32.h clearly indicates
that it's meant to have the same license as crc32.c.  Therefore, apply
SPDX-License-Identifier: GPL-2.0-only to both files.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250514052409.194822-1-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@google.com>