PCI: Restore inclusion of pci/hotplug Kconfig

Commit e7e127e ("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 cfeb813 ("Merge branch 'pci/host-hv' into
next") merge.  "git diff cfeb813^ cfeb813" 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>

drivers/pci/Kconfig

@@ -128,4 +128,5 @@ config PCI_HYPERV
           The PCI device frontend driver allows the kernel to import arbitrary
           PCI devices from a PCI backend to support PCI driver domains.
 
+source "drivers/pci/hotplug/Kconfig"
 source "drivers/pci/host/Kconfig"