ARM: configs: libretech-ac: enable USB_DFU like in meson64.h

USB_DFU was added in meson64.h but is missing in libretech-ac.h,
fix this to enable DFU RAM boot for libretech-ac.

Fixes 4aa027b3 ("configs: meson64: add alternate USB DFU boot target")

Link: https://lore.kernel.org/r/20231102-libretech-ac-fix-dfu-v1-1-112379165028@linaro.org


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>