several netfilter compilation units rely on implicit includes
coming from nf_conntrack_proto_gre.h.
Clean this up and add the required dependencies where needed.
nf_conntrack.h requires net_generic() helper.
Place various gre/ppp/vlan includes to where they are needed.
Signed-off-by: Florian Westphal <fw@strlen.de>
d00453b6e3