media: docs: v4l2-controls: document file to include

The tutorial in this section is almost complete, add the one missing bit.
Signed-off-by: NLuca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: NHans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab+samsung@kernel.org>

Documentation/media/kapi/v4l2-controls.rst

@@ -50,6 +50,10 @@ Basic usage for V4L2 and sub-device drivers
 
 1) Prepare the driver:
 
+.. code-block:: c
+
+	#include <media/v4l2-ctrls.h>
+
 1.1) Add the handler to your driver's top-level struct:
 
 For V4L2 drivers: