Impact: fix compile error on s390.

With "cpumask: update irq_desc to use cpumask_var_t" in linux-next
we get this compile bug on s390:

  CC      drivers/char/sysrq.o
In file included from drivers/char/sysrq.c:38:
include/linux/irq.h: In function 'init_alloc_desc_masks':
include/linux/irq.h:442: error: dereferencing pointer to incomplete type

Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMike Travis <travis@sgi.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>