local.conf.sample: add a commented-out option to enable the gtk UI qemu frontend

Gtk is disabled by default as SDL is somewhat quicker to build.

(From meta-yocto rev: d48f63bba98c94171db5da826c35c50ff638d2fb)

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>