libnetfilter_queue: does not build without IPv6 support

Fixes the following build failure:

extra/checksum.c:16:25: fatal error: netinet/ip6.h: No such file or directory
compilation terminated.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>