PCI/ASPM: Unexport pcie_aspm_support_enabled()

pcie_aspm_support_enabled() is used only by the acpi/pci_root.c driver,
which cannot be built as a module, so it does not need to be exported.
Unexport it.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>