PCI: Make bus_attr_resource_alignment static

The symbol bus_attr_resource_alignment is not exported or declared
elsewhere, so make it static to fix the following warning:

  drivers/pci/pci.c:4900:1: warning: symbol 'bus_attr_resource_alignment' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>