[pci] Drag in PCI settings mechanism only when PCI support is present

Allow for the existence of platforms with no PCI bus by including the
PCI settings mechanism only if PCI bus support is included.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/bus/pci.c

@@ -438,3 +438,9 @@ struct root_device pci_root_device __root_device = {
 	.dev = { .name = "PCI" },
 	.driver = &pci_root_driver,
 };
+
+/* Drag in objects via pcibus_probe() */
+REQUIRING_SYMBOL ( pcibus_probe );
+
+/* Drag in PCI configuration */
+REQUIRE_OBJECT ( config_pci );