webkitgtk: security bump to version 2.18.6

webkitgtk: security bump to version 2.18.6

This is a maintenance release of the current stable WebKitGTK+ version,
which contains security fixes for CVE-2018-4088, CVE-2017-13885,
CVE-2017-7165, CVE-2017-13884, CVE-2017-7160, CVE-2017-7153,
CVE-2017-7153, CVE-2017-7161, and CVE-2018-4096. Additionally, it solves
a GStreamer deadlock when stopping video playback, and contains fixes
and improvements for the WebDriver implementation.

Release notes can be found in the announcement:

  https://webkitgtk.org/2018/01/24/webkitgtk2.18.6-released.html

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>