of_serial: explicitly include of_irq.h

On PPC, of_irq.h gets implicitly included, but on other platforms
it does not.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/serial/of_serial.c

@@ -15,6 +15,7 @@
 #include <linux/serial_core.h>
 #include <linux/serial_8250.h>
 #include <linux/of_address.h>
+#include <linux/of_irq.h>
 #include <linux/of_platform.h>
 #include <linux/nwpserial.h>