package/libnetfilter_conntrack: add libmnl to Libs.Private

This will fix a static build failure with dnsmasq on latest
libnetfilter_conntrack

Fixes:
 - http://autobuild.buildroot.org/results/3fdc2cba20162eb86eaa5c49a056fb40fb18a392



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Tested-by: Baruch Siach <baruch@tkos.co.il>
[Peter: adjust upstream status as pointed out by Baruch]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>