staging: greybus: fix more header declarations

More headers needed to be fixed when moving greybus out of staging and
enabling the COMPILE_TEST option.

Add forward declarations for the needed structures.

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20190828124825.20800-1-rui.silva@linaro.org


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