mlx5e: add CONFIG_INET dependency

We now reference the arp_tbl, which requires IPv4 support to be
enabled in the kernel, otherwise we get a link error:

drivers/net/built-in.o: In function `mlx5e_tc_update_neigh_used_value':
(.text+0x16afec): undefined reference to `arp_tbl'
drivers/net/built-in.o: In function `mlx5e_rep_neigh_init':
en_rep.c:(.text+0x16c16d): undefined reference to `arp_tbl'
drivers/net/built-in.o: In function `mlx5e_rep_netevent_event':
en_rep.c:(.text+0x16cbb5): undefined reference to `arp_tbl'

This adds a Kconfig dependency for it.

Fixes: 232c0013 ("net/mlx5e: Add support to neighbour update flow")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>