PCI/AER: Make <linux/aer.h> standalone includable

The header file references u16 and u32 types, but they are not defined in
the header nor does the header pull in the necessary includes for them.
This causes build breakage when the file is included without any of the
dependencies being satisfied from somewhere else.

Fix this by including linux/types.h (for u16 and u32).

[bhelgaas: removed pci_dev declaration (already added by 5ccb8225)]
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>