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: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>