netfilter: xt_TEE: use IS_ENABLED(CONFIG_NF_DUP_IPV6)

Instead of IS_ENABLED(CONFIG_IPV6), otherwise we hit:

et/built-in.o: In function `tee_tg6':
>> xt_TEE.c:(.text+0x6cd8c): undefined reference to `nf_dup_ipv6'

when:

 CONFIG_IPV6=y
 CONFIG_NF_DUP_IPV4=y
 # CONFIG_NF_DUP_IPV6 is not set
 CONFIG_NETFILTER_XT_TARGET_TEE=y

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>