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.home
Signed-off-by: Ingo Molnar <mingo@kernel.org>