genirq: Add missing irq_to_desc export for CONFIG_SPARSE_IRQ=n

In allmodconfig builds for sparc and any other arch which does
not set CONFIG_SPARSE_IRQ, the following will be seen at modpost:

  CC [M]  lib/cpu-notifier-error-inject.o
  CC [M]  lib/pm-notifier-error-inject.o
ERROR: "irq_to_desc" [drivers/gpio/gpio-mcp23s08.ko] undefined!
make[2]: *** [__modpost] Error 1

This happens because commit 3911ff30 ("genirq: export
handle_edge_irq() and irq_to_desc()") added one export for it, but
there were actually two instances of it, in an if/else clause for
CONFIG_SPARSE_IRQ.  Add the second one.
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: stable@vger.kernel.org	# 3.4+
Link: http://lkml.kernel.org/r/1392057610-11514-1-git-send-email-paul.gortmaker@windriver.comSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/irq/irqdesc.c

@@ -274,6 +274,7 @@ struct irq_desc *irq_to_desc(unsigned int irq)
 {
 	return (irq < NR_IRQS) ? irq_desc + irq : NULL;
 }
+EXPORT_SYMBOL(irq_to_desc);
 
 static void free_desc(unsigned int irq)
 {