Add enable_ozone_wayland_vkb=1 to GYP_DEFINES.

This allows to enable virtual keyboard on Tizen IVI.
Disable xdg_shell by default.

This patch disables xdg_shell by default as we support ver1 only and
the available weston might not support the given version. One can enable
support by having GYP_DEFINE enable_xdg_shell set to 1.