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>