Fix macOS build.

  system/core/adb/client/usb_libusb.cpp:162:20: error: unused function 'get_device_serial_path' [-Werror,-Wunused-function]
  static std::string get_device_serial_path(libusb_device* device) {
                     ^

Bug: N/A
Test: no choice but to submit and see...
Change-Id: I0026d3273891588c27df5ccd71c41ad9907b038d