webkitgtk: security bump to version 2.12.4

Fixes:
CVE-2016-4590 - mishandles about: URLs, which allows remote attackers to
bypass the Same Origin Policy via a crafted web site.

CVE-2016-4591 - mishandles the location variable, which allows remote
attackers to access the local filesystem via unspecified vectors.

CVE-2016-4622 - allows remote attackers to execute arbitrary code or
cause a denial of service (memory corruption) via a crafted web site, a
different vulnerability than CVE-2016-4589, CVE-2016-4623, and
CVE-2016-4624.

CVE-2016-4624 - allows remote attackers to execute arbitrary code or
cause a denial of service (memory corruption) via a crafted web site, a
different vulnerability than CVE-2016-4589, CVE-2016-4622, and
CVE-2016-4623.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>