hw/s390x: Include missing 'cpu.h' header

hw/s390x: Include missing 'cpu.h' header

"cpu.h" is implicitly included. Include it explicitly to
avoid the following error when refactoring headers:

  hw/s390x/s390-stattrib.c:86:40: error: use of undeclared identifier 'TARGET_PAGE_SIZE'
      len = sac->peek_stattr(sas, addr / TARGET_PAGE_SIZE, buflen, vals);
                                         ^
  hw/s390x/s390-stattrib.c:94:58: error: use of undeclared identifier 'TARGET_PAGE_MASK'
                     addr / TARGET_PAGE_SIZE, len, addr & ~TARGET_PAGE_MASK);
                                                         ^
  hw/s390x/s390-stattrib.c:224:40: error: use of undeclared identifier 'TARGET_PAGE_BITS'
          qemu_put_be64(f, (start_gfn << TARGET_PAGE_BITS) | STATTR_FLAG_MORE);
                                         ^
  In file included from hw/s390x/s390-virtio-ccw.c:17:
  hw/s390x/s390-virtio-hcall.h:22:27: error: unknown type name 'CPUS390XState'
  int s390_virtio_hypercall(CPUS390XState *env);
                            ^

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Eric Farman <farman@linux.ibm.com>
Message-ID: <20240322162822.7391-1-philmd@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>