apparmor: add #ifdef checks for secmark filtering

The newly added code fails to build when either SECMARK or
NETFILTER are disabled:

security/apparmor/lsm.c: In function 'apparmor_socket_sock_rcv_skb':
security/apparmor/lsm.c:1138:12: error: 'struct sk_buff' has no member named 'secmark'; did you mean 'mark'?

security/apparmor/lsm.c:1671:21: error: 'struct nf_hook_state' declared inside parameter list will not be visible outside of this definition or declaration [-Werror]

Add a set of #ifdef checks around it to only enable the code that
we can compile and that makes sense in that configuration.

Fixes: ab9f2115

 ("apparmor: Allow filtering based on secmark policy")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: John Johansen <john.johansen@canonical.com>