hw/acpi: Include "hw/mem/nvdimm.h"

Both ich9.c and piix4.c use methods/definitions declared in the
NVDIMM device header. Include it.

This fixes (when modifying unrelated headers):

  hw/acpi/ich9.c:507:46: error: use of undeclared identifier 'TYPE_NVDIMM'
        if (object_dynamic_cast(OBJECT(dev), TYPE_NVDIMM)) {
                                             ^
  hw/acpi/ich9.c:508:13: error: implicit declaration of function 'nvdimm_acpi_plug_cb' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
            nvdimm_acpi_plug_cb(hotplug_dev, dev);
            ^
  hw/acpi/piix4.c:403:46: error: use of undeclared identifier 'TYPE_NVDIMM'
        if (object_dynamic_cast(OBJECT(dev), TYPE_NVDIMM)) {
                                             ^
  hw/acpi/piix4.c:404:13: error: implicit declaration of function 'nvdimm_acpi_plug_cb' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
            nvdimm_acpi_plug_cb(hotplug_dev, dev);
            ^

Acked-by: John Snow <jsnow@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200228114649.12818-16-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>