Hack up distcheck for publican

Publican requires a read-write source tree, see
http://bugzilla.redhat.com/show_bug.cgi?id=798484



And it currently cannot build out-of-tree, so we need to copy the sources
into the _build tree and generate Protocol.xml into that tree too (we'd have
to do this anyway since automake creates a read-only source tree, so we
can't just link).

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>