babeld: address some compilation warnings

babeld: address some compilation warnings

Including system headers is not necessary with zebra.h included and
sometimes results in "__ASSERT_FUNCTION redefined" compilation warning.

* babeld.c
  * babel_distribute_update_interface(): make static
* babel_interface.c
  * interface_config_write(): unused 'babel_ifp'
  * don't include system headers
* message.c
  * send_request(): unused 'babel_ifp'
  * send_multihop_request(): idem
  * don't include system headers
* route.c: don't include system headers
* xroute.c: idem
* source.h: newline at EOF
* message.h: idem