PCI: Fix missing prototype for pcie_port_acpi_setup()

This fixes a "no previous prototype" warning for pcie_port_acpi_setup()
found via "make W=1".

[bhelgaas: changelog]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>