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>

scripts/headerdep.pl

@@ -19,7 +19,7 @@ my $opt_graph;
 	version	=> \&version,
 
 	all	=> \$opt_all,
-	I	=> \@opt_include,
+	"I=s"	=> \@opt_include,
 	graph	=> \$opt_graph,
 );