librsvg: security bump to version 2.40.18
http://ftp.gnome.org/pub/gnome/sources/librsvg/2.40/librsvg-2.40.18.news
CVE-2017-11464 - Fix division-by-zero in the Gaussian blur code.
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>