commit ae34fa900b introduced a new
function nm_netlink_route6_add(). Its declaration in the header
includes a parameter with type 'struct in6_addr', but the proper
system header for that type was not included.
Add two helper functions, one for IPv4 and one for IPv6, to ensure
that the core code benefits from compiler type checking when adding
routes. Previously nm_netlink_route_add() took a void* which meant
we messed up adding IPv6 routes sometimes due to confusion over
what was supposed to be passed to it. Also fixes what appears to
be a C&P error with add_ip6_route_to_gateway().
Reported by Tomáš Trnka <tomastrnka@gmx.com>
