package/libvncserver: disable gtk example
gtkvncviewer has been added since version 0.9.13 and
https://github.com/LibVNC/libvncserver/commit/2650cfc17bb7718e42bfff8510c15f011eda1b1f,
disable it as it is only an example
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>