On FDT systems, gpiobus.c depends on ofw_gpiobus_register_provider() and
ofw_gpiobus_unregister_provider() from ofw_gpiobus.c; this file is included
in the kernel build if 'device gpio' is enabled, but is not inluded in the
Apologies for asking for a review of a three-liner; I just didn't want to
stomp on anything outside my purview.