kbuild: fix single *.ko build

The single *.ko build is broken since commit f65a4868 ("kbuild:
change module.order to list *.o instead of *.ko").

Fixes: f65a4868

 ("kbuild: change module.order to list *.o instead of *.ko")
Reported-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Marc Kleine-Budde <mkl@pengutronix.de>