Move at-spi dbus xml definitions into platformsupport, these files
simply describe the DBus protocol and should not be in 3rdparty.

The header files can just as well be picked up by pkg-config.

Change-Id: I326d9b3cb69223bf2c8646099f211d9a9f3fa0af
Reviewed-by: Lars Knoll <lars.knoll@qt.io>