ipv6: hide ip6_encap_hlen/ip6_tnl_encap definitions

A recent cleanup moved MAX_IPTUN_ENCAP_OPS along with some other
definitions, but it is now invisible when CONFIG_INET is
not defined, but still referenced from ip6_tunnel.h:

In file included from net/xfrm/xfrm_input.c:17:0:
include/net/ip6_tunnel.h:67:17: error: 'MAX_IPTUN_ENCAP_OPS' undeclared here (not in a function)
   ip6tun_encaps[MAX_IPTUN_ENCAP_OPS];
                 ^~~~~~~~~~~~~~~~~~~

This hides the ip6_encap_hlen and ip6_tnl_encap functions inside
of CONFIG_INET so we don't run into the the problem.

Alternatively we could move the macro out of the #ifdef again to
restore the previous behavior

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 55c2bc14

 ("net: Cleanup encap items in ip_tunnels.h")
Signed-off-by: David S. Miller <davem@davemloft.net>