ACPI / PCI: Include appropriate header file in pci_slot.c

Includes appropriate header file linux/pci-acpi.h in pci_slot.c
because functions acpi_pci_slot_enumerate(), acpi_pci_slot_remove()
and acpi_pci_slot_init() have their prototype declaratons in
linux/pci-acpi.h.

This eliminates the following warnings in pci_slot.c:
drivers/acpi/pci_slot.c:162:6: warning: no previous prototype for ‘acpi_pci_slot_enumerate’ [-Wmissing-prototypes]
drivers/acpi/pci_slot.c:174:6: warning: no previous prototype for ‘acpi_pci_slot_remove’ [-Wmissing-prototypes]
drivers/acpi/pci_slot.c:215:13: warning: no previous prototype for ‘acpi_pci_slot_init’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>