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>

drivers/xen/Kconfig

@@ -64,7 +64,7 @@ config XEN_SYS_HYPERVISOR
 
 config XEN_PLATFORM_PCI
 	tristate "xen platform pci device driver"
-	depends on XEN_PVHVM
+	depends on XEN_PVHVM && PCI
 	default m
 	help
 	  Driver for the Xen PCI Platform device: it is responsible for