media: v4l: async: Add V4L2 async documentation to the documentation build

The V4L2 async wasn't part of the documentation build. Fix this.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/kapi/v4l2-async.rst

+V4L2 async kAPI
+^^^^^^^^^^^^^^^
+.. kernel-doc:: include/media/v4l2-async.h

Documentation/media/kapi/v4l2-core.rst

@@ -19,6 +19,7 @@ Video4Linux devices
     v4l2-mc
     v4l2-mediabus
     v4l2-mem2mem
+    v4l2-async
     v4l2-fwnode
     v4l2-rect
     v4l2-tuner