hns3: prevent building without CONFIG_INET

We now get a link failure when CONFIG_INET is disabled, since
tcp_gro_complete is unavailable:

drivers/net/ethernet/hisilicon/hns3/hns3_enet.o: In function `hns3_set_gro_param':
hns3_enet.c:(.text+0x230c): undefined reference to `tcp_gro_complete'

Add an explicit CONFIG_INET dependency here to avoid the broken
configuration.

Fixes: a6d53b97

 ("net: hns3: Adds GRO params to SKB for the stack")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>