arch-x86: include system syscall header in syscall table files

arch-x86: include system syscall header in syscall table files

The getdents syscall is only implemented on hosts that define
SYS_getdents, which is located in <sys/syscall.h>.

That header was missed when splitting the syscall tables into their own
files; this patch adds the header to the syscall table files.

Change-Id: I28d54f6ea2874aa533c89ed7520561e19fe5e5f9
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37195
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>