Accessibility Linux: Do not depend on external libatspi2

Accessibility Linux: Do not depend on external libatspi2

This has (and still does) caused lots of grief since
it means accessibility was often unintendedly not built.

Instead copy the lib-at-spi-2 header file needed for the
type enum and build it by default again.

Change-Id: I1ba26f20edff1aeb444c96a37928f36230ac7576
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>