netfilter: add missing IS_ENABLED(CONFIG_NF_TABLES) check to header-file.

commit 47e640af

 upstream.

nf_tables.h defines an API comprising several inline functions and
macros that depend on the nft member of struct net.  However, this is
only defined is CONFIG_NF_TABLES is enabled.  Added preprocessor checks
to ensure that nf_tables.h will compile if CONFIG_NF_TABLES is disabled.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Link: https://lore.kernel.org/netfilter-devel/20190920094925.aw7actk4tdnk3rke@salvia/T/
Fixes: 3c171f49

 ("netfilter: bridge: add connection tracking system")
Reported-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>