net: Make IPv6 build depend on CONFIG_INET

IPv6 build selection currently controlled by CONFIG_NET, so it is
possible to build IPv6 functinality without selectiona any TCP/IP
features (CONFIG_INET).  Make IPv6 be consistent with IPv4.

This should resolve the following issue:

net/built-in.o: In function `tcp6_gro_complete':
tcpv6_offload.c:(.text+0x3d045): undefined reference to
`tcp_gro_complete'
net/built-in.o: In function `tcp6_gro_receive':
tcpv6_offload.c:(.text+0x3d19b): undefined reference to
`tcp_gro_receive'
net/built-in.o: In function `ipv6_exthdrs_offload_init':
(.init.text+0x118b): undefined reference to `inet_del_offload'
net/built-in.o:(.rodata+0x1c58): undefined reference to
`tcp_tso_segment'

Signed-off-by: Vlad Yasevich <vyasvic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>