marge-bot enabled on the xserver repo

[Peter Hutterer]

Just a heads-up, I've enabled marge-bot on the xserver repo so from now
on please assign to @marge-bot to get the MR to merge which will then do
the right thing (whatever it already does in mesa and other projects,
it's all the same commandline anyway).

Advantages include easier merging of multiple MRs (no more waiting) and
a backlink to the MR (Part-Of: ) for future reference. Those two were
really the seller for me.

Cheers,
  Peter