Fix gcc build error while CONFIG_INET is not set

drivers/net/ethernet/stmicro/stmmac/stmmac_selftests.o: In function `__stmmac_test_loopback':
stmmac_selftests.c:(.text+0x8ec): undefined reference to `ip_send_check'
stmmac_selftests.c:(.text+0xacc): undefined reference to `udp4_hwcsum'

Add CONFIG_INET dependency to fix this.
Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 091810db ("net: stmmac: Introduce selftests support")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>