PCI: Restore inclusion of pci/hotplug Kconfig

Commit e7e127e3 ("PCI: Include pci/hotplug Kconfig directly from
pci/Kconfig") added one line to pci/Kconfig.  However, for some mysterious
reason it isn't there now, even though there are no traces of removing it
in the git log.

I detected this issue when 'make oldconfig' removed all the options that
depended on HOTPLUG_PCI.

[bhelgaas: I botched the cfeb8139 ("Merge branch 'pci/host-hv' into
next") merge.  "git diff cfeb8139^ cfeb8139" shows a conflict in
drivers/pci/Kconfig, and I mistakenly dropped the hotplug/Kconfig piece.]
Signed-off-by: Tero Roponen <tero.roponen@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>