kbuild: add comment about grouped target

GNU Make commit 8c888d95f618 ("[SV 8297] Implement "grouped targets"
for explicit rules.") added the '&:' syntax.

I think '&:' is a perfect fit here, but we cannot use it any time
soon. Just add a TODO comment.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>