webkitgtk: security bump to version 2.18.3

webkitgtk: security bump to version 2.18.3

This is a maintenance release of the current stable WebKitGTK+ version,
which contains a minor rendering fix, another for the WebDriver
implementation, and security fixes for CVE-2017-13798, CVE-2017-13788,
and CVE-2017-13803.

Release notes:

    https://webkitgtk.org/2017/11/10/webkitgtk2.18.3-released.html

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit 5ff18880e9647e446a3d720b7c6c74eed97ef0b4)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>