nftables: fix static build

libnftnl needs -ljansson when built with json support.

Fixes:
http://autobuild.buildroot.net/results/68a/68ab2101a0b2b7d5482aafe0d88c87a50491159f/
http://autobuild.buildroot.net/results/87e/87ecbba5b9b16d592a48fe827fa88ef5a5c6d09b/

libnftl also needs -mxml when built with XML support.

Fixes:
http://autobuild.buildroot.net/results/58f/58f592d823db14e4e7558134f1b479f4f8659a23/
http://autobuild.buildroot.net/results/1cf/1cf23f2b38236f43971c65e704793b5bc762ecfe/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>