webkitgtk: security bump to version 2.18.5

webkitgtk: security bump to version 2.18.5

This is a maintenance release of the current stable WebKitGTK+ version,
which contains mitigations for CVE-2017-5753 and CVE-2017-5715, the
vulnerabilities known as the "Spectre" attack. It also contains a fix
which allows building the reference documentation with newer gtk-doc
versions.

Release notes can be found in the announcement:

  https://webkitgtk.org/2018/01/10/webkitgtk2.18.5-released.html

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