babeld: adopt to upstream header-style (#640)

babeld: adopt to upstream header-style (#640)

babeld: adopt to upstream header-style

Instead of including the headerfiles that define the structs, we add
forward definitions to our headerfile.

Fixes warning:

ubus.h:67:32: warning: 'struct xroute' declared inside parameter list will not be visible outside of this definition or declaration
 void ubus_notify_xroute(struct xroute *xroute, int kind);
                                ^~~~~~
Signed-off-by: Nick Hainke <vincent@systemli.org>