xen/acpi: remove redundant acpi/acpi_drivers.h include

It's redundant since linux/acpi.h has include it when CONFIG_ACPI enabled,
and when CONFIG_ACPI disabled it will trigger compiling warning

In file included from drivers/xen/xen-stub.c:28:0:
include/acpi/acpi_drivers.h:103:31:
warning: 'struct acpi_device' declared inside parameter list [enabled by default]
include/acpi/acpi_drivers.h:103:31:
warning: its scope is only this definition or declaration, which is probably not what you want [enabled by default]
include/acpi/acpi_drivers.h:107:43:
warning: 'struct acpi_pci_root' declared inside parameter list [enabled by default]

Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Liu Jinsong <jinsong.liu@intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>