update-linux-headers.sh: unistd.h, kvm consistency

update-linux-headers.sh: unistd.h, kvm consistency

Rework the update script slightly, add the unistd.h header and its
dependencies on all architectures.

This also removes the IA64 and MIPS from a KVM blacklist:
Linux dropped IA64, and there was never a reason to
exclude MIPS from kvm specifically - it was
excluded due to dependency of its unistd.h on sgidefs.h,
which we also import.

Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>