apparmor: depends on NET

SECURITY_APPARMOR should depend on NET since AUDIT needs
(depends on) NET.

Fixes 70-80 errors that occur when CONFIG_NET is not enabled,
but APPARMOR selects AUDIT without qualification.  E.g.:

audit.c:(.text+0x33361): undefined reference to `netlink_unicast'
(.text+0x333df): undefined reference to `netlink_unicast'
audit.c:(.text+0x3341d): undefined reference to `skb_queue_tail'
audit.c:(.text+0x33424): undefined reference to `kfree_skb'
audit.c:(.text+0x334cb): undefined reference to `kfree_skb'
audit.c:(.text+0x33597): undefined reference to `skb_put'
audit.c:(.text+0x3369b): undefined reference to `__alloc_skb'
audit.c:(.text+0x336d7): undefined reference to `kfree_skb'
(.text+0x3374c): undefined reference to `__alloc_skb'
auditfilter.c:(.text+0x35305): undefined reference to `skb_queue_tail'
lsm_audit.c:(.text+0x2873): undefined reference to `init_net'
lsm_audit.c:(.text+0x2878): undefined reference to `dev_get_by_index'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: James Morris <jmorris@namei.org>