adwaita-icon-theme: depends on host-libgtk3

The adwaita icon theme can only be selected with libgtk3 and depends on
host-libgtk3 to build.

Fixes http://autobuild.buildroot.org/results/f8d/f8d0e2198fa54f9141512e2db6dc46e3cee1cd44/



Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>