CONNSECMARK needs conntrack, add missing dependency to fix linking error
with CONNSECMARK=y and CONNTRACK=m.

Reported by Toralf Frster <toralf.foerster@gmx.de>.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>