PCI: drivers/pci/slot.c should depend on CONFIG_SYSFS

There is no way to interact with a physical PCI slot without
sysfs, so encode the dependency and prevent this build error:

	drivers/pci/slot.c: In function 'pci_hp_create_module_link':
	drivers/pci/slot.c:327: error: 'module_kset' undeclared

This patch _should_ make pci-sysfs.o depend on CONFIG_SYSFS too,
but we cannot (yet) because the PCI core merrily assumes the
existence of sysfs:

	drivers/built-in.o: In function `pci_bus_add_device':
	drivers/pci/bus.c:89: undefined reference to `pci_create_sysfs_dev_files'
	drivers/built-in.o: In function `pci_stop_dev':
	drivers/pci/remove.c:24: undefined reference to `pci_remove_sysfs_dev_files'

So do the minimal bit for now and figure out how to untangle it
later.

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fix-suggested-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>