coccinelle: api/atomic_as_refcounter: add SPDX License Identifier

Add the missing GPLv2 SPDX license identifier.

It appears this single file was missing from 7f904d7e

 ("treewide:
Replace GPLv2 boilerplate/reference with SPDX - rule 505"), which
addressed all other files in scripts/coccinelle. Hence I added
GPL-2.0-only consitently with the mentioned patch.

Cc: linux-spdx@vger.kernel.org
Cc: Elena Reshetova <elena.reshetova@intel.com>
Signed-off-by: Matthias Maennich <maennich@google.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>