usb: gadget: include <linux/prefetch.h> to fix compiling error

drivers/usb/gadget/at91_udc.c: In function 'write_fifo':
drivers/usb/gadget/at91_udc.c:421:2: error: implicit declaration of function 'prefetch'
make[3]: *** [drivers/usb/gadget/at91_udc.o] Error 1
make[2]: *** [drivers/usb/gadget] Error 2
make[2]: *** Waiting for unfinished jobs....

Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
diff --git a/drivers/usb/gadget/s3c2410_udc.c b/drivers/usb/gadget/s3c2410_udc.c
index 6d8b040..100f263 100644
--- a/drivers/usb/gadget/s3c2410_udc.c
+++ b/drivers/usb/gadget/s3c2410_udc.c
@@ -36,6 +36,7 @@
 #include <linux/platform_device.h>
 #include <linux/clk.h>
 #include <linux/gpio.h>
+#include <linux/prefetch.h>
 
 #include <linux/debugfs.h>
 #include <linux/seq_file.h>