at-spi2-core: disable API docs if x11 is disabled

The gtk-doc documentation doesn't build if X11 is disabled.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>