arch-x86: Add sys/syscall.h to x86 process.cc/syscall_emul.cc

arch-x86: Add sys/syscall.h to x86 process.cc/syscall_emul.cc

Change a66d12c guards the selection of getdents()
in x86's process.cc file with SYS_getdents, however
process.cc does not include the right header for
SYS_getdents, which leads to x86 choosing the
unimplemented call. This change adds sys/syscall.h to
address the problem.

This change also adds sys/syscall.hh to syscall_emu.cc,
which only includes syscall.hh and may not be supported on
all systems.

Change-Id: If1adcf41e9e455de5f2827ba98c542fdcacdc22e
Reviewed-on: https://gem5-review.googlesource.com/c/14775
Reviewed-by: Ciro Santilli <ciro.santilli@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>