kbuild: fix option processing for -I in headerdep

-I takes an argument.  Without this change only a 1 is added to
@opt_include which is not helpful.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>