sparseirq: fix build with unknown irq_desc struct

Ingo Molnar wrote:
>
> tip/kernel/fork.c: In function 'copy_signal':
> tip/kernel/fork.c:825: warning: unused variable 'ret'
> tip/drivers/char/random.c: In function 'get_timer_rand_state':
> tip/drivers/char/random.c:584: error: dereferencing pointer to incomplete type
> tip/drivers/char/random.c: In function 'set_timer_rand_state':
> tip/drivers/char/random.c:594: error: dereferencing pointer to incomplete type
> make[3]: *** [drivers/char/random.o] Error 1

irq_desc is defined in linux/irq.h, so include it in the genirq case.
Signed-off-by: NYinghai Lu <yinghai@kernel.org>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

drivers/char/random.c

@@ -241,6 +241,10 @@
 #include <linux/percpu.h>
 #include <linux/cryptohash.h>
 
+#ifdef CONFIG_GENERIC_HARDIRQS
+# include <linux/irq.h>
+#endif
+
 #include <asm/processor.h>
 #include <asm/uaccess.h>
 #include <asm/irq.h>