genirq: Export dummy_irq_chip

Export dummy_irq_chip to modules to allow them to do things such as

	irq_set_chip_and_handler(virq,
				 &dummy_irq_chip,
				 handle_level_irq);
This fixes

	ERROR: "dummy_irq_chip" [drivers/gpio/gpio-pcf857x.ko] undefined!

when gpio-pcf857x.c is being built as a module.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Greg KH <gregkh@linuxfoundation.org>
Link: http://lkml.kernel.org/r/871ujstrp6.wl%25kuninori.morimoto.gx@renesas.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/irq/dummychip.c

@@ -6,6 +6,7 @@
  */
 #include <linux/interrupt.h>
 #include <linux/irq.h>
+#include <linux/export.h>
 
 #include "internals.h"
 
@@ -57,3 +58,4 @@ struct irq_chip dummy_irq_chip = {
 	.irq_mask	= noop,
 	.irq_unmask	= noop,
 };
+EXPORT_SYMBOL_GPL(dummy_irq_chip);