genirq: Fix UP compile failure caused by irq_thread_check_affinity

Since genirq: Delegate irq affinity setting to the irq thread
(591d2fb0) compilation with
CONFIG_SMP=n fails with following error:

/usr/src/linux-2.6/kernel/irq/manage.c:
   In function 'irq_thread_check_affinity':
/usr/src/linux-2.6/kernel/irq/manage.c:475:
   error: 'struct irq_desc' has no member named 'affinity'
make[4]: *** [kernel/irq/manage.o] Error 1

That commit adds a new function irq_thread_check_affinity() which
uses struct irq_desc.affinity which is only available for CONFIG_SMP=y.
Move that function under #ifdef CONFIG_SMP.

[ tglx@brownpaperbag: compile and boot tested on UP and SMP ]

Signed-off-by: Bruno Premont <bonbons@linux-vserver.org>
LKML-Reference: <20090722222232.2eb3e1c4@neptune.home>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>