x86/PCI: only define pci_domain_nr if PCI and PCI_DOMAINS are set

Otherwise we'll duplicate definitions with the pci.h stubs.

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>