webkitgtk: update to 2.18.5 (includes Spectre mitigations; see commit description)

webkitgtk: update to 2.18.5 (includes Spectre mitigations; see commit description)

This is the only available stable version with mitigation fixes for Spectre.
Webkit upstream developers do not port CVE fixes to earlier stable series,
no exception was made in this case.

More information:

https://webkit.org/blog/8048/what-spectre-and-meltdown-mean-for-webkit/
https://webkitgtk.org/security/WSA-2018-0001.html
https://webkitgtk.org/2018/01/10/webkitgtk2.18.5-released.html

Signed-off-by: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>