irq_work.h: fix warning when CONFIG_IRQ_WORK=n

A randconfig caught repeated compiler warnings when CONFIG_IRQ_WORK=n
due to the definition of a non-inline static function in
<linux/irq_work.h>:

include/linux/irq_work.h +40 : warning: 'irq_work_needs_cpu' defined but not used

Make it inline to supress the warning. This is caused commit
00b42959106a ("irq_work: Don't stop the tick with pending works") merged
in v3.9-rc1.

Signed-off-by: James Hogan <[e-mail]>
Signed-off-by: Frederic Weisbecker <[e-mail]>
Cc: Steven Rostedt <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Paul Gortmaker <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/irq_work.h

···
 #ifdef CONFIG_IRQ_WORK
 bool irq_work_needs_cpu(void);
 #else
-static bool irq_work_needs_cpu(void) { return false; }
+static inline bool irq_work_needs_cpu(void) { return false; }
 #endif
 
 #endif /* _LINUX_IRQ_WORK_H */