mail-client/evolution-3.32.4: Don't allow webkitgtk-2.26

mail-client/evolution-3.32.4: Don't allow webkitgtk-2.26

This version would be broken (nearly invisible text in messages)
with webkit-gtk-2.26, so don't allow it.
The evolution-3.32.5 bump should work better with webkit-gtk-2.26.
https://gitlab.gnome.org/GNOME/evolution/issues/617

However webkit-gtk-2.26 still would be breaking evolution-3.32.5
too, but that's a separate battle - one which might end up as
a workaround in the webkit-gtk-2.26 bump instead.

Package-Manager: Portage-2.3.79, Repoman-2.3.12
Signed-off-by: Mart Raudsepp <leio@gentoo.org>