irq_work: fix compile failure on tile from missing include

Building with IRQ_WORK configured results in

kernel/irq_work.c: In function ‘irq_work_run’:
kernel/irq_work.c:110: error: implicit declaration of function ‘irqs_disabled’

The appropriate header just needs to be included.
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

kernel/irq_work.c

@@ -11,6 +11,7 @@
 #include <linux/irq_work.h>
 #include <linux/percpu.h>
 #include <linux/hardirq.h>
+#include <linux/irqflags.h>
 #include <asm/processor.h>
 
 /*