configs: enable CONFIG_NET_FOU_IP_TUNNELS

The current configs enable fou as a module, but fou is not very
useful without CONFIG_NET_FOU_IP_TUNNELS, because it can't be
used to actually create tunnels:

$ sudo ip fou add port 12345 ipproto ipip
$ sudo ip link add name fou1 type ipip local any remote 8.8.8.8 \
      encap fou encap-sport auto encap-dport 12345
RTNETLINK answers: Invalid argument

Enable the option by default. This does not increase memory
footprint because fou is not loaded by default. Uncompressed
module size goes from 26.3KiB to 32.5KiB.

After this change, things work:

$ sudo ip fou add port 12345 ipproto ipip
$ sudo ip link add name fou1 type ipip local any remote 8.8.8.8 \
      encap fou encap-sport auto encap-dport 12345
$ ip link show fou1
8: fou1@NONE: <POINTOPOINT,NOARP> mtu 1472 qdisc noop state DOWN [...]
    link/ipip 0.0.0.0 peer 8.8.8.8

Signed-off-by: Lorenzo Colitti <lorenzo@colitti.com>