webkitgtk: security bump to version 2.18.2

This is a maintenance release of the current stable WebKitGTK+ version,
which contains bugfixes; mostly for crashes and rendering issues, plus
one important fix for the layout or Arabic text.

Release notes:

    https://webkitgtk.org/2017/10/27/webkitgtk2.18.2-released.html



Even though an acconpanying security advisory has not been published
for this release, the release contains fixes for several crashes (one
of them for the decoder of the very common GIF image format), which
arguably can be considered potential security issues.

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