libgtk3: build gtk-encode-symbolic-svg for host variant

libgtk3: build gtk-encode-symbolic-svg for host variant

It's required to create the symbolic icons (RGBA/transparent) which some
apps like to use, like connman-gtk.
The added patch avoids the need for a full host-libgtk3 build which would
require host-libepoxy, host-mesa3d and a ton other deps.
And pull in host-librsvg so that we have the rsvg gdk-pixbuf loader
(plugin), otherwise scalable (svg) icons will error when trying to be
processed, like those provided by adwaita-icon-theme.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>