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
00b42959

 ("irq_work: Don't stop the tick with pending works") merged
in v3.9-rc1.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>