vim: disable GTK+3 UI by default

vim: disable GTK+3 UI by default

Disable the GTK+ UI by default.  By having vim be always built with GTK+
by default installing vim results in the entire GTK+/X11 stack being
pulled into images, even if they would otherwise be non-graphical.

Also, real vim users run it in a terminal.

The PACKAGECONFIG remains and can be enabled. Maybe a future improvement
would be to split the recipe into non-X vim and GTK+-based gvim recipes.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>