rcu/tree: Export rcu_idle_{enter,exit} to modules

Fix this link error:

ERROR: modpost: "rcu_idle_enter" [drivers/acpi/processor.ko] undefined!
ERROR: modpost: "rcu_idle_exit" [drivers/acpi/processor.ko] undefined!

when CONFIG_ACPI_PROCESSOR is built as module. PeterZ says that in light
of ARM needing those soon too, they should simply be exported.

Fixes: 1fecfdbb7acc ("ACPI: processor: Take over RCU-idle for C3-BM idle")
Reported-by: Sven Joachim <[e-mail]>
Suggested-by: Peter Zijlstra <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Reviewed-by: Paul E. McKenney <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

kernel/rcu/tree.c

···
 	lockdep_assert_irqs_disabled();
 	rcu_eqs_enter(false);
 }
+EXPORT_SYMBOL_GPL(rcu_idle_enter);
 
 #ifdef CONFIG_NO_HZ_FULL
 /**
···
 	rcu_eqs_exit(false);
 	local_irq_restore(flags);
 }
+EXPORT_SYMBOL_GPL(rcu_idle_exit);
 
 #ifdef CONFIG_NO_HZ_FULL
 /**