act_connmark: fix dependencies better

NET_ACT_CONNMARK fails to build if NF_CONNTRACK_MARK is disabled,
and d7924450 ("act_connmark: Add missing dependency on
NF_CONNTRACK_MARK") fixed that case, but missed the cased where
NF_CONNTRACK is a loadable module.

This adds the second dependency to ensure that NET_ACT_CONNMARK
can only be built-in if NF_CONNTRACK is also part of the kernel
rather than a loadable module.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>