kbuild: use POSIX-compatible grep option

--file is a GNU extension to grep which is not available in all
implementations (such as BusyBox). Use the -f option instead which is
eqvuialent according to the GNU grep manpage[1] and is present in
POSIX[2].

 [1] https://www.gnu.org/software/grep/manual/grep.html
 [2] https://pubs.opengroup.org/onlinepubs/9699919799/utilities/grep.html



Signed-off-by: Stefan Hansson <newbie13xd@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>