netfilter: nf_tables: fix ifdef to also consider nf_tables=m

nftables can be built as a module, so fix the preprocessor conditional
accordingly.

Fixes: 478b360a ("netfilter: nf_tables: fix nf_trace always-on with XT_TRACE=n")
Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Reported-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>