My future plan is to list subdirectories in ./Kbuild. When it occurs,
$(vmlinux-alldirs) will not contain all subdirectories.

Let's hard-code the directory list until I get around to implementing
a more sophisticated way for generating a source tarball.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>