Fix a openvswitch compilation error when CONFIG_INET is not set:

=====================================================
   In file included from include/net/geneve.h:4:0,
                       from net/openvswitch/flow_netlink.c:45:
		          include/net/udp_tunnel.h: In function 'udp_tunnel_handle_offloads':
			  >> include/net/udp_tunnel.h:100:2: error: implicit declaration of function 'iptunnel_handle_offloads' [-Werror=implicit-function-declaration]
			  >>      return iptunnel_handle_offloads(skb, udp_csum, type);
			  >>           ^
			  >>           >> include/net/udp_tunnel.h:100:2: warning: return makes pointer from integer without a cast
			  >>           >>    cc1: some warnings being treated as errors

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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>