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

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>

include/net/netfilter/nf_tables.h

@@ -1207,6 +1207,8 @@ void nft_trace_notify(struct nft_traceinfo *info);
 #define MODULE_ALIAS_NFT_OBJ(type) \
 	MODULE_ALIAS("nft-obj-" __stringify(type))
 
+#if IS_ENABLED(CONFIG_NF_TABLES)
+
 /*
  * The gencursor defines two generations, the currently active and the
  * next one. Objects contain a bitmask of 2 bits specifying the generations
@@ -1280,6 +1282,8 @@ static inline void nft_set_elem_change_active(const struct net *net,
 	ext->genmask ^= nft_genmask_next(net);
 }
 
+#endif /* IS_ENABLED(CONFIG_NF_TABLES) */
+
 /*
  * We use a free bit in the genmask field to indicate the element
  * is busy, meaning it is currently being processed either by