PCI: cpcihp: Declare cpci_debug in header file

PCI: cpcihp: Declare cpci_debug in header file

cpci_debug is declared as a global variable in cpci_hotplug_core.c and used
in cpci_hotplug_pci.c via an "extern".  Add an extern declaration in the
header file.

Resolves the following sparse warning:

  drivers/pci/hotplug/cpci_hotplug_core.c:47:5: warning: symbol 'cpci_debug' was not declared. Should it be static?

Link: https://lore.kernel.org/r/20210701184306.1492003-1-kw@linux.com
Signed-off-by: Krzysztof Wilczyński <kw@linux.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>