demo/gtk_client: Fix GSourceFunc compiler warning (cast)

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>
Conditionally build bxt_gtk_client

The builder must now pass --enable-gtk-demo as well as --enable-demos
if they wish to build the GTK3 demo client. This is to ensure that
GTK3 and its dependencies are not required merely to build the
default demonstrations.

Signed-off-by: Michael Ikey Doherty <michael.i.doherty@intel.com>