xen-platform: Fix compile errors if CONFIG_PCI is not enabled.

drivers/xen/platform-pci.c:127: error: implicit declaration of function
          'pci_request_region'
drivers/xen/platform-pci.c:165: error: implicit declaration of function
          'pci_release_region'

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>