xen/pci: Fix build on non-x86

We can't include <asm/pci_x86.h> if this isn't x86, and we only need
it if CONFIG_PCI_MMCONFIG is enabled.

Fixes: 8deb3eb1 ('xen/mcfg: Call PHYSDEVOP_pci_mmcfg_reserved for MCFG areas.')
Signed-off-by: NBen Hutchings <ben@decadent.org.uk>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: NDavid Vrabel <david.vrabel@citrix.com>
Acked-by: NIan Campbell <ian.campbell@citrix.com>

drivers/xen/pci.c

@@ -26,7 +26,9 @@
 #include <asm/xen/hypervisor.h>
 #include <asm/xen/hypercall.h>
 #include "../pci/pci.h"
+#ifdef CONFIG_PCI_MMCONFIG
 #include <asm/pci_x86.h>
+#endif
 
 static bool __read_mostly pci_seg_supported = true;