netfilter: fix undefined reference to 'netfilter_lwtunnel_*' when CONFIG_SYSCTL=n

if CONFIG_SYSFS is not enabled in config, we get the below compile error,

All errors (new ones prefixed by >>):

   csky-linux-ld: net/netfilter/core.o: in function `netfilter_init':
   core.c:(.init.text+0x42): undefined reference to `netfilter_lwtunnel_init'
>> csky-linux-ld: core.c:(.init.text+0x56): undefined reference to `netfilter_lwtunnel_fini'
>> csky-linux-ld: core.c:(.init.text+0x70): undefined reference to `netfilter_lwtunnel_init'
   csky-linux-ld: core.c:(.init.text+0x78): undefined reference to `netfilter_lwtunnel_fini'

Fixes: a2225e02 ("netfilter: move the sysctl nf_hooks_lwtunnel into the netfilter core")
Reported-by: Mirsad Todorovac <mtodorovac69@gmail.com>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406210511.8vbByYj3-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202406210520.6HmrUaA2-lkp@intel.com/Signed-off-by: Jianguo Wu <wujianguo@chinatelecom.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_hooks_lwtunnel.c

@@ -117,4 +117,7 @@ void netfilter_lwtunnel_fini(void)
 {
 	unregister_pernet_subsys(&nf_lwtunnel_net_ops);
 }
+#else
+int __init netfilter_lwtunnel_init(void) { return 0; }
+void netfilter_lwtunnel_fini(void) {}
 #endif /* CONFIG_SYSCTL */