genirq: Add prototype for handle_nested_irq()

The function is supposed to be called from the primary IRQ
handler for a demultiplexing chip so make a protype visible for
them.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
LKML-Reference: <1251142084-9852-1-git-send-email-broonie@opensource.wolfsonmicro.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>