net/udp.h is currently included indirectly via linux/ipv6.h which is
in turn included indirectly via linux/ip.h. However, this breaks down
if CONFIG_IPV6 is not set, leading to a number of build errors.
Signed-off-by: Simon Horman <horms@verge.net.au>
[Jesse: shortened commit message]
Signed-off-by: Jesse Gross <jesse@nicira.com>