USB: r8a66597-udc: Prototype IS_ERR() and PTR_ERR()

The build of r8a66597-udc was failing on ARM since IS_ERR() and
PTR_ERR() weren't protyped.  Presumably err.h is being pulled in by
another header on other platforms.
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Yoshihiro Shimoda <shimoda.yoshihiro@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/gadget/r8a66597-udc.c

@@ -26,6 +26,7 @@
 #include <linux/io.h>
 #include <linux/platform_device.h>
 #include <linux/clk.h>
+#include <linux/err.h>
 
 #include <linux/usb/ch9.h>
 #include <linux/usb/gadget.h>