usb/gadget: aspeed-vhub: add USB_LIBCOMPOSITE dependency

Without that option, we run into a link failure:

drivers/usb/gadget/udc/aspeed-vhub/hub.o: In function `ast_vhub_std_hub_request':
hub.c:(.text+0x5b0): undefined reference to `usb_gadget_get_string'

Fixes: 7ecca2a4 ("usb/gadget: Add driver for Aspeed SoC virtual hub")
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>