secmark: fix config problem when CONFIG_NF_CONNTRACK_SECMARK is not set

When CONFIG_NF_CONNTRACK_SECMARK is not set we accidentally attempt to use
the secmark fielf of struct nf_conn.  Problem is when that config isn't set
the field doesn't exist.  whoops.  Wrap the incorrect usage in the config.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>