SubmitChecklist: interfaces changes should CC linux-api@

Mention that patches that change the kernel-userland interface should
be CCed to the new list linux-api@vger.kernel.org.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>