PCI: make pci_ltr_supported() static

The PCI Express Latency Tolerance Reporting (LTR) feature's
pci_ltr_supported() routine is currently only used within
drivers/pci/pci.c so make it static.

Acked-by: Donald Dutile <ddutile@redhat.com>
Signed-off-by: Myron Stowe <myron.stowe@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>