PCI: Change pci_host_common_probe() visibility

pci_host_common_probe() is defined when CONFIG_PCI_HOST_COMMON=y;
therefore the function declaration should match that.

  drivers/pci/host/pcie-tango.c:300:9: error:
	implicit declaration of function 'pci_host_common_probe'

Signed-off-by: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>