Nothing in ipv6.h needs ndisc.h, drop it.

Link: https://lore.kernel.org/r/20220203043457.2222388-1-kuba@kernel.orgAcked-by: Jeremy Kerr <jk@codeconstruct.com.au>
Acked-by: Stefan Schmidt <stefan@datenfreihafen.org>
Link: https://lore.kernel.org/r/20220203231240.2297588-1-kuba@kernel.orgReviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>