mptcp: MPTCP_IPV6 should depend on IPV6 instead of selecting it

mptcp: MPTCP_IPV6 should depend on IPV6 instead of selecting it

MPTCP_IPV6 selects IPV6, thus enabling an optional feature the user may
not want to enable.  Fix this by making MPTCP_IPV6 depend on IPV6, like
is done for all other IPv6 features.

Fixes: bc796e574a6e91e8 ("mptcp: Add MPTCP socket stubs")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20201020073839.29226-1-geert@linux-m68k.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>