lib/asan.c: Update SPDX license

lint-000-license-headers reports error.
The SPDX identifier contains GPL-2.0

Update the identifier to GPL-2.0-only.

BUG = N/A
TEST = Build Intel Elkhart Lake

Change-Id: If49fd014f14b481163bca6cd3131139b6d95c6d8
Signed-off-by: Frans Hendriks <fhendriks@eltan.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50109
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/lib/asan.c

@@ -1,4 +1,4 @@
-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: GPL-2.0-only */
 
 /*
  * Address sanitizer support.