arch/syscall-defs.h: Add missing SPDX-License-Identifier
arch/syscall-defs.h: Add missing SPDX-License-Identifier

This adds missing SPDX-License-Identifier for:
  src/arch/syscall-defs.h

Signed-off-by: Ammar Faizi <a8266392a329a5c7b5ab982a8996391d75b6438e@gnuweeb.org>

axboe/liburing,axboe/liburing,axboe/liburing