route/mdb: cleanup mdb.h header
route/mdb: cleanup mdb.h header

- drop leading tabs. Yes, it's inside an extern C block, but so is all
  our code in public headers. That should not by default increase the
  indentation level.

- drop unused rtnl_mdb_add().

- forward declare "struct rtnl_mdb".

greearb/libnl-ct,thom311/libnl,thom311/libnl,greearb/libnl-ct,thom311/libnl,greearb/libnl-ct