Force to add missing rtk_btusb.ko, which is ignored by .gitignore

Missing device/softwinner/common/hardware/realtek/bluetooth/firmware/rtl8723au/rtk_btusb.ko cause error:

make: *** No rule to make target `device/softwinner/common/hardware/realtek/bluetooth/firmware/rtl8723au/rtk_btusb.ko', needed by `out/target/product/wing-mbox203/system/vendor/modules/rtk_btusb.ko'.