PCI/AER: Remove "extern" from function declarations

We had an inconsistent mix of using and omitting the "extern" keyword
on function declarations in header files.  This removes them all.

[bhelgaas: split out from "move AER severity defines" patch]
Signed-off-by: Betty Dall <betty.dall@hp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>