genirq: provide irq_to_desc() to non-genirq architectures too

Impact: build fix on non-genirq architectures

Sam Ravnborg reported this build failure on sparc32 allmodconfig,
the GPIO drivers assume the presence of irq_to_desc():

 drivers/gpio/gpiolib.c: In function `gpiolib_dbg_show':
 drivers/gpio/gpiolib.c:1146: error: implicit declaration of function 'irq_to_desc'

Add it in the !genirq case too.

Reported-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Tested-by: Sam Ravnborg <sam@ravnborg.org>