sphinx.rst: describe the install requirements for kfigure

As we now have a document describing the install
requirements for Sphinx, add there the need for GraphViz
and ImageMagick.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
1 file changed