netfilter: ctnetlink: Guard possible unused functions

Some of the functions may be unused (CONFIG_NETFILTER_NETLINK_GLUE_CT=n
and CONFIG_NF_CONNTRACK_EVENTS=n), it prevents kernel builds with clang,
`make W=1` and CONFIG_WERROR=y:

net/netfilter/nf_conntrack_netlink.c:657:22: error: unused function 'ctnetlink_acct_size' [-Werror,-Wunused-function]
  657 | static inline size_t ctnetlink_acct_size(const struct nf_conn *ct)
      |                      ^~~~~~~~~~~~~~~~~~~
net/netfilter/nf_conntrack_netlink.c:667:19: error: unused function 'ctnetlink_secctx_size' [-Werror,-Wunused-function]
  667 | static inline int ctnetlink_secctx_size(const struct nf_conn *ct)
      |                   ^~~~~~~~~~~~~~~~~~~~~
net/netfilter/nf_conntrack_netlink.c:683:22: error: unused function 'ctnetlink_timestamp_size' [-Werror,-Wunused-function]
  683 | static inline size_t ctnetlink_timestamp_size(const struct nf_conn *ct)
      |                      ^~~~~~~~~~~~~~~~~~~~~~~~

Fix this by guarding possible unused functions with ifdeffery.

See also commit 6863f564 ("kbuild: allow Clang to find unused static
inline functions for W=1 build").
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_netlink.c

@@ -652,7 +652,6 @@ static size_t ctnetlink_proto_size(const struct nf_conn *ct)
 
 	return len + len4;
 }
-#endif
 
 static inline size_t ctnetlink_acct_size(const struct nf_conn *ct)
 {
@@ -690,6 +689,7 @@ static inline size_t ctnetlink_timestamp_size(const struct nf_conn *ct)
 	return 0;
 #endif
 }
+#endif
 
 #ifdef CONFIG_NF_CONNTRACK_EVENTS
 static size_t ctnetlink_nlmsg_size(const struct nf_conn *ct)