webkitgtk: security bump to version 2.18.1

webkitgtk: security bump to version 2.18.1

This is a maintenance release of the current stable WebKitGTK+ version,
which contains bugfixes (many of them related to rendering, plus one
important fix for touch input) and many security fixes.

Release notes:

    https://webkitgtk.org/2017/10/18/webkitgtk2.18.1-released.html

Fixes CVE-2017-7081, CVE-2017-7087, CVE-2017-7089, CVE-2017-7090,
CVE-2017-7091, CVE-2017-7092, CVE-2017-7093, CVE-2017-7094,
CVE-2017-7095, CVE-2017-7096, CVE-2017-7098, CVE-2017-7099,
CVE-2017-7100, CVE-2017-7102, CVE-2017-7104, CVE-2017-7107,
CVE-2017-7109, CVE-2017-7111, CVE-2017-7117, CVE-2017-7120,
CVE-2017-7142:

    https://webkitgtk.org/security/WSA-2017-0008.html

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>