PCI hotplug: SGI hotplug: fix build failure

The commit bd3d99c1 ("PCI: Remove
untested Electromechanical Interlock (EMI) support in pciehp."), which
removes the definition of "struct hotplug_slot_attr", broke SGI
hotplug driver. By this commit, we get the following compile error.

drivers/pci/hotplug/sgi_hotplug.c:106: error: variable 'sn_slot_path_attr' has initializer but incomplete type
drivers/pci/hotplug/sgi_hotplug.c:106: error: unknown field 'attr' specified in initializer
drivers/pci/hotplug/sgi_hotplug.c:106: error: extra brace group at end of initializer
drivers/pci/hotplug/sgi_hotplug.c:106: error: (near initialization for 'sn_slot_path_attr')
drivers/pci/hotplug/sgi_hotplug.c:106: warning: excess elements in struct initializer
drivers/pci/hotplug/sgi_hotplug.c:106: warning: (near initialization for 'sn_slot_path_attr')
drivers/pci/hotplug/sgi_hotplug.c:106: error: unknown field 'show' specified in initializer
drivers/pci/hotplug/sgi_hotplug.c:106: warning: excess elements in struct initializer
drivers/pci/hotplug/sgi_hotplug.c:106: warning: (near initialization for 'sn_slot_path_attr')
drivers/pci/hotplug/sgi_hotplug.c: In function 'sn_hp_destroy':
drivers/pci/hotplug/sgi_hotplug.c:203: error: invalid use of undefined type 'struct hotplug_slot_attribute'
drivers/pci/hotplug/sgi_hotplug.c: In function 'sn_hotplug_slot_register':
drivers/pci/hotplug/sgi_hotplug.c:655: error: invalid use of undefined type 'struct hotplug_slot_attribute'

This patch fixes this regression by adding the definition of struct
hotplug_slot_attr into sgi_hotplug.c.

Tested-by: Mike Habeck <habeck@sgi.com>
Signed-off-by: Kenji Kaneshige <kaneshige.kenji@jp.fujitsu.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>