Include required ofw_gpiobus.c when building gpiobus module for OFW/FDT targets.

[landonf]

Summary

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
module build.

Apologies for asking for a review of a three-liner; I just didn't want to
stomp on anything outside my purview.

[jhibbits]

Already done, in r327681.

[landonf]

Whoops, I got beat to it :-)