kbuild: don't hardcode depmod path

commit 436e980e upstream.

depmod is not guaranteed to be in /sbin, just let make look for
it in the path like all the other invoked programs

Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>