Fix build when CONFIG_IPV6 is not enabled by making a function
be built conditionally.

Fixes these build errors and warnings:

../drivers/net/ethernet/mellanox/mlx5/core/en_accel/fs_tcp.c: In function 'accel_fs_tcp_set_ipv6_flow':
../include/net/sock.h:380:34: error: 'struct sock_common' has no member named 'skc_v6_daddr'; did you mean 'skc_daddr'?
  380 | #define sk_v6_daddr  __sk_common.skc_v6_daddr
      |                                  ^~~~~~~~~~~~
../drivers/net/ethernet/mellanox/mlx5/core/en_accel/fs_tcp.c:55:14: note: in expansion of macro 'sk_v6_daddr'
   55 |         &sk->sk_v6_daddr, 16);
      |              ^~~~~~~~~~~
At top level:
../drivers/net/ethernet/mellanox/mlx5/core/en_accel/fs_tcp.c:47:13: warning: 'accel_fs_tcp_set_ipv6_flow' defined but not used [-Wunused-function]
   47 | static void accel_fs_tcp_set_ipv6_flow(struct mlx5_flow_spec *spec, struct sock *sk)

Fixes: 5229a96e59ec ("net/mlx5e: Accel, Expose flow steering API for rules add/del")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>