netronome: fix ipv6 link error

When the driver is built-in but ipv6 is a module, the flower
support produces a link error:

drivers/net/ethernet/netronome/nfp/flower/tunnel_conf.o: In function `nfp_tunnel_keep_alive_v6':
tunnel_conf.c:(.text+0x2aa8): undefined reference to `nd_tbl'

Add a Kconfig dependency to avoid that configuration.

Fixes: 9ea9bfa1 ("nfp: flower: support ipv6 tunnel keep-alive messages from fw")
Acked-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>