of/irq: of_irq.c needs to include linux/irq.h

It works on current architectures simply because asm/prom.h includes
it, but it broke when x86 turned on CONFIG_OF.
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

include/linux/of_irq.h

@@ -5,6 +5,7 @@
 struct of_irq;
 #include <linux/types.h>
 #include <linux/errno.h>
+#include <linux/irq.h>
 #include <linux/ioport.h>
 #include <linux/of.h>