PCI: Add GPL license for drivers/pci/ioapic module

Config PCI_IOAPIC turned into a tristate in commit b95a7bd7, but no
module license is specified.  This adds the missing module license.

Signed-off-by: Andrew Cooks <acooks@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jan Beulich <jbeulich@suse.com>