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

drivers/usb/misc/onboard_usb_hub_pdevs.c

@@ -15,6 +15,7 @@
 #include <linux/usb.h>
 #include <linux/usb/hcd.h>
 #include <linux/usb/of.h>
+#include <linux/usb/onboard_hub.h>
 
 #include "onboard_usb_hub.h"