udp_tunnel: Only build ip6_udp_tunnel.c when IPV6 is selected

Functions supplied in ip6_udp_tunnel.c are only needed when IPV6 is
selected. When IPV6 is not selected, those functions are stubbed out
in udp_tunnel.h.

==================================================================
 net/ipv6/ip6_udp_tunnel.c:15:5: error: redefinition of 'udp_sock_create6'
     int udp_sock_create6(struct net *net, struct udp_port_cfg *cfg,
 In file included from net/ipv6/ip6_udp_tunnel.c:9:0:
      include/net/udp_tunnel.h:36:19: note: previous definition of 'udp_sock_create6' was here
       static inline int udp_sock_create6(struct net *net, struct udp_port_cfg *cfg,
==================================================================

Fixes:  fd384412

 udp_tunnel: Seperate ipv6 functions into its own file
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>