Fix build error when CONFIG_ACPI is not set/enabled:

../arch/x86/pci/xen.c: In function ‘pci_xen_init’:
../arch/x86/pci/xen.c:410:2: error: implicit declaration of function ‘acpi_noirq_set’; did you mean ‘acpi_irq_get’? [-Werror=implicit-function-declaration]
  acpi_noirq_set();

Fixes: 88e9ca16 ("xen/pci: Use acpi_noirq_set() helper to avoid #ifdef")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: xen-devel@lists.xenproject.org
Cc: linux-pci@vger.kernel.org
Signed-off-by: Juergen Gross <jgross@suse.com>

arch/x86/pci/xen.c

@@ -26,6 +26,7 @@
 #include <asm/xen/pci.h>
 #include <asm/xen/cpuid.h>
 #include <asm/apic.h>
+#include <asm/acpi.h>
 #include <asm/i8259.h>
 
 static int xen_pcifront_enable_irq(struct pci_dev *dev)