scripts/update-linux-headers.sh: adjust for Linux 4.21-rc1 (or 5.0-rc1)

scripts/update-linux-headers.sh: adjust for Linux 4.21-rc1 (or 5.0-rc1)

There are three new indirect inclusions: vhost_types.h, which we'll
shortly put to use as a portable header and thus is copied to
standard-headers; and new per-subtarget versions of MIPS unistd.h
and PowerPC unistd.h.

Because vhost.h includes vhost_types.h, we also need a proxy include
from linux/vhost.h to standard-headers.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>