dbus: Put DTDs into $out instead of $doc

First of all, these "documents" are not really documentation, so it
really doesn't make much sense to put it into $doc.

The main point however is that the installer tests are failing since
this was introduced in ac0cdc19

.

One way to circumvent this is putting dbus.doc into
system.extraDependencies of the installer tests, but given the first
point this sounds a bit odd to me.

So I went for the second way of putting it into $out, because it's now
basically necessary to build a NixOS system.

With this the NixOS installer tests should now work again, although I
have only tested this with the installer.simple test.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @abbradar