PCI: Make pci_enable_ptm() private

This interface:

  int pci_enable_ptm(struct pci_dev *dev, u8 *granularity);

is only used in drivers/pci/ and does not need to be seen by the rest of
the kernel.  Move it to drivers/pci/pci.h so it's private to the PCI
subsystem.

Link: https://lore.kernel.org/r/20190724233848.73327-11-skunberg.kelsey@gmail.com
Signed-off-by: Kelsey Skunberg <skunberg.kelsey@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>