sphinx-pre-install: fix USE needs for GraphViz and ImageMagick

Gentoo need some USE for GraphViz and ImageMagick to have
the features required by kfigure.py.

Output that when providing instructions for Gentoo.

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