usb: misc: onboard_hub: Fix 'missing prototype' warning

When building with 'W=1' the compiler complains about missing
prototypes for onboard_hub_create/destroy_pdevs(). Include the
header with the prototypes to fix this.

Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Link: https://lore.kernel.org/r/20220711092431.1.I4016c759fd7fe2b32dd482994a20661f36e2cae3@changeid


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>