kernel: fix two implicit header assumptions in irq_work.c

Up until now, this file was getting percpu.h because nearly every
file was implicitly getting module.h (and all its sub-includes).
But we want to clean that up, so call out percpu.h explicitly.
Otherwise we'll get things like this on an ARM build:

kernel/irq_work.c:48: error: expected declaration specifiers or '...' before 'irq_work_list'
kernel/irq_work.c:48: warning: type defaults to 'int' in declaration of 'DEFINE_PER_CPU'

The same thing was happening for builds on ARM for asm/processor.h

kernel/irq_work.c: In function 'irq_work_sync':
kernel/irq_work.c:166: error: implicit declaration of function 'cpu_relax'
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

kernel/irq_work.c

@@ -8,7 +8,9 @@
 #include <linux/kernel.h>
 #include <linux/export.h>
 #include <linux/irq_work.h>
+#include <linux/percpu.h>
 #include <linux/hardirq.h>
+#include <asm/processor.h>
 
 /*
  * An entry can be in one of four states: