sparse irqs: handle !GENIRQ platforms

Impact: build fix

fix:

 In file included from /home/mingo/tip/arch/m68k/amiga/amiints.c:39:
 /home/mingo/tip/include/linux/interrupt.h:21: error: expected identifier or '('
 /home/mingo/tip/arch/m68k/amiga/amiints.c: In function 'amiga_init_IRQ':

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