netfilter: add missing IS_ENABLED(CONFIG_NF_CONNTRACK) checks to some header-files.

struct nf_conn contains a "struct nf_conntrack ct_general" member and
struct net contains a "struct netns_ct ct" member which are both only
defined in CONFIG_NF_CONNTRACK is enabled.  These members are used in a
number of inline functions defined in other header-files.  Added
preprocessor checks to make sure the headers will compile if
CONFIG_NF_CONNTRACK is disabled.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>