PCI/AER: Change pci_aer_init() stub to return void

Commit 60ed982a ("PCI/AER: Move internal declarations to
drivers/pci/pci.h") changed pci_aer_init() to return "void", but didn't
change the stub for when CONFIG_PCIEAER isn't enabled.  Change the stub to
match.

Fixes: 60ed982a ("PCI/AER: Move internal declarations to drivers/pci/pci.h")
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: stable@vger.kernel.org	# v4.19+