irq_work: Fix build failure when CONFIG_IRQ_WORK is not defined

When CONFIG_IRQ_WORK is not defined (difficult to do, as it also
requires CONFIG_PRINTK not to be defined), we get a build failure:

	kernel/built-in.o: In function `flush_smp_call_function_queue':
	kernel/smp.c:263: undefined reference to `irq_work_run'
	kernel/smp.c:263: undefined reference to `irq_work_run'
	Makefile:933: recipe for target 'vmlinux' failed

Simplest thing to do is to make irq_work_run() a nop when not set.
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/20150319101851.4d224d9b@gandalf.local.homeSigned-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/irq_work.h

@@ -38,16 +38,17 @@ bool irq_work_queue(struct irq_work *work);
 bool irq_work_queue_on(struct irq_work *work, int cpu);
 #endif
 
-void irq_work_run(void);
 void irq_work_tick(void);
 void irq_work_sync(struct irq_work *work);
 
 #ifdef CONFIG_IRQ_WORK
 #include <asm/irq_work.h>
 
+void irq_work_run(void);
 bool irq_work_needs_cpu(void);
 #else
 static inline bool irq_work_needs_cpu(void) { return false; }
+static inline void irq_work_run(void) { }
 #endif
 
 #endif /* _LINUX_IRQ_WORK_H */