sphinx-pre-install: add dependencies for ImageMagick to work with svg

ImageMagick actually uses librsvg for conversions when converiting
from SVG (actually, it uses rsvg-convert). That causes the build to
fail with:

	WARNING: Error #1 when calling: /usr/bin/convert /home/mchehab/docs/Documentation/media/uapi/v4l/selection.svg /home/mchehab/docs/Documentation/output/latex/selection.pdf
	convert: delegate failed `'rsvg-convert' -o '%o' '%i'' @ error/delegate.c/InvokeDelegate/1919.
	convert: unable to open file `/tmp/magick-8883oOQfHzrA5trM': No such file or directory @ error/constitute.c/ReadImage/544.

Add the corresponding dependencies.

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