musb: fix prefetch build failure

After the prefetch/list.h restructure, drivers need to explicitly include
linux/prefetch.h in order to use the prefetch() function.  Otherwise, the
current driver fails to build:

drivers/usb/musb/musb_core.c: In function 'musb_write_fifo':
drivers/usb/musb/musb_core.c:219: error: implicit declaration of function 'prefetch'
make[3]: *** [drivers/usb/musb/musb_core.o] Error 1
Signed-off-by: NMike Frysinger <vapier@gentoo.org>
Signed-off-by: NFelipe Balbi <balbi@ti.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

drivers/usb/musb/musb_core.c

@@ -96,6 +96,7 @@
 #include <linux/init.h>
 #include <linux/list.h>
 #include <linux/kobject.h>
+#include <linux/prefetch.h>
 #include <linux/platform_device.h>
 #include <linux/io.h>