smack: dont compile ipv6 code unless ipv6 is configured

I want to be sure that ipv6-specific code
is not compiled in kernel binaries
if ipv6 is not configured.

[1] was getting rid of "unused variable" warning, but,
with that, it also mandated compilation of a handful ipv6-
specific functions in ipv4-only kernel configurations:

smk_ipv6_localhost, smack_ipv6host_label, smk_ipv6_check.

Their compiled bodies are likely to be removed by compiler
from the resulting binary, but, to be on the safe side,
I remove them from the compiler view.

[1]
Fixes: 00720f0e ("smack: avoid unused 'sip' variable warning")

Signed-off-by: Konstantin Andreev <andreev@swemel.ru>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>