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

kernel/irq/irqdesc.c

@@ -352,7 +352,9 @@ struct irq_desc *irq_to_desc(unsigned int irq)
 {
 	return radix_tree_lookup(&irq_desc_tree, irq);
 }
-EXPORT_SYMBOL(irq_to_desc);
+#ifdef CONFIG_KVM_BOOK3S_64_HV
+EXPORT_SYMBOL_GPL(irq_to_desc);
+#endif
 
 static void delete_irq_desc(unsigned int irq)
 {