webkitgtk: security bump to version 2.12.3

Fixes:
CVE-2016-1856 - allows remote attackers to execute arbitrary code or
cause a denial of service (memory corruption) via a crafted web site
CVE-2016-1857 - allows remote attackers to execute arbitrary code or
cause a denial of service (memory corruption) via a crafted web site

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>