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: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NAndy Zhou <azhou@nicira.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/Makefile

@@ -35,7 +35,6 @@ obj-$(CONFIG_INET6_XFRM_MODE_ROUTEOPTIMIZATION) += xfrm6_mode_ro.o
 obj-$(CONFIG_INET6_XFRM_MODE_BEET) += xfrm6_mode_beet.o
 obj-$(CONFIG_IPV6_MIP6) += mip6.o
 obj-$(CONFIG_NETFILTER)	+= netfilter/
-obj-$(CONFIG_NET_UDP_TUNNEL) += ip6_udp_tunnel.o
 
 obj-$(CONFIG_IPV6_VTI) += ip6_vti.o
 obj-$(CONFIG_IPV6_SIT) += sit.o
@@ -46,3 +45,7 @@ obj-y += addrconf_core.o exthdrs_core.o ip6_checksum.o ip6_icmp.o
 obj-$(CONFIG_INET) += output_core.o protocol.o $(ipv6-offload)
 
 obj-$(subst m,y,$(CONFIG_IPV6)) += inet6_hashtables.o
+
+ifneq ($(CONFIG_IPV6),)
+obj-$(CONFIG_NET_UDP_TUNNEL) += ip6_udp_tunnel.o
+endif