net/mlx5e: add IPV6 dependency

The ethernet support now calls directly into the ipv6 core code, which
fails if IPV6 is a loadable module but mlx5 is built-in:

drivers/net/ethernet/mellanox/mlx5/core/en_tc.o: In function `mlx5e_create_encap_header_ipv6':
en_tc.c:(.text.mlx5e_create_encap_header_ipv6+0x110): undefined reference to `ip6_route_output_flags'

This adds a dependency to ensure that MLX5_CORE_EN can only be built
if we are able link the kernel successfully. The downside is that the
ethernet option can be hidden. Alternatively we could make MLX5_CORE
depend on "IPV6 || !IPV6", which would force MLX5_CORE to be a module
when IPV6 is, including in configurations where we don't use the ethernet
support at all.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>