Fix build.

Before, at-spi2-core installed accessibility.conf in $(sysconfdir)/at-spi2,
so we used the CONF_FILES mechanism to place it in
share/examples/at-spi2-core/at-spi2, so the administrator can make local
adjustments.

Now, at-spi2-core works with sensible defaults without an
accessibility.conf file, and places it in $(datadir)/defaults/at-spi2,
essentially to be used as an example.

Choose to revert to the old behaviour.

(prlw1)