um: Add missing EXPORT for free_irq_by_fd()

Fixes:
ERROR: "free_irq_by_fd" [arch/um/drivers/random.ko] undefined!
Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/kernel/irq.c

@@ -328,6 +328,7 @@ void free_irq_by_fd(int fd)
 	garbage_collect_irq_entries();
 	spin_unlock_irqrestore(&irq_lock, flags);
 }
+EXPORT_SYMBOL(free_irq_by_fd);
 
 static void free_irq_by_irq_and_dev(unsigned int irq, void *dev)
 {