PCI: Move cpci_hotplug_init() proto to header file

cpci_hotplug_init() and cpci_hotplug_exit() are defined in
cpci_hotplug_core.c but had extern declarations in pci_hotplug_core.c.
This puts the declarations in a header file included both places so
the compiler can help keep everything consistent.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>