irq: export __set_irq_handler() and handle_level_irq()

Impact: build fix

ARM updates broke x86 allmodconfig builds:

 ERROR: "__set_irq_handler" [drivers/mfd/pcf50633-core.ko] undefined!
 ERROR: "handle_level_irq" [drivers/mfd/pcf50633-core.ko] undefined!

Signed-off-by: Ingo Molnar <mingo@elte.hu>