ppp: enable ipv6 support

networkmanager requires pppd to have ipv6 support for managing mobile
connections since version 0.9.5.95, ref. to commit 7575f4d [1] or mailinglist [2].

i also thought about setting it up as an option which is only pulled in by nm, but i don't see enough benefit for the added complexity.

footnotes:

[1] http://cgit.freedesktop.org/NetworkManager/NetworkManager/commit/?id=7575f4d1e451a006e4ddd9cc6caa664ce07df83f

[2] https://mail.gnome.org/archives/networkmanager-list/2012-October/msg00053.html