lkdtm: Add missing SPDX-License-Identifier line

The refcount.c file missed the mass-addition of the SPDX lines.
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/lkdtm/refcount.c

+// SPDX-License-Identifier: GPL-2.0
 /*
  * This is for all the tests related to refcount bugs (e.g. overflow,
  * underflow, reaching zero untested, etc).