genirq: Restrict export of irq_to_desc()

No more (ab)use in drivers finally. There is still the modular build of
PPC/KVM which needs it, so restrict it to this case which still makes it
unavailable for most drivers.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201210194045.551428291@linutronix.de