irq_work: Define irq_work_single() on !CONFIG_IRQ_WORK too

Some SMP platforms don't have CONFIG_IRQ_WORK defined, resulting in a link
error at build time.

Define a stub and clean up the prototype definitions.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

kernel/smp.c

@@ -194,8 +194,6 @@ void generic_smp_call_function_single_interrupt(void)
 	flush_smp_call_function_queue(true);
 }
 
-extern void irq_work_single(void *);
-
 /**
  * flush_smp_call_function_queue - Flush pending smp-call-function callbacks
  *