sparc64: Fix build by including linux/irq.h into time_64.c

Changeset d7e51e66899f95dabc89b4d4c6674a6e50fa37fc ("sparseirq: make
some func to be used with genirq") broke the build on sparc64:

arch/sparc/kernel/time_64.c: In function ‘timer_interrupt’:
arch/sparc/kernel/time_64.c:732: error: implicit declaration of function ‘kstat_incr_irqs_this_cpu’
make[1]: *** [arch/sparc/kernel/time_64.o] Error 1

Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>