bpf: Define sock security related BTF IDs under CONFIG_SECURITY_NETWORK

There are warnings reported from resolve_btfids when building vmlinux
with CONFIG_SECURITY_NETWORK disabled:

  WARN: resolve_btfids: unresolved symbol bpf_lsm_sk_free_security
  WARN: resolve_btfids: unresolved symbol bpf_lsm_sk_alloc_security

So only define BTF IDs for these LSM hooks when CONFIG_SECURITY_NETWORK
is enabled.

Fixes: c0c852dd

 ("bpf: Do not mark certain LSM hook arguments as trusted")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221217062144.2507222-1-houtao@huaweicloud.com