meson: ensure correct build order of VAPI
Without this commit spice-client-gtk-3.0.vapi may be built before spice-client-glib-2.0.vapi if build_jobs > 1. This causes the build to fail because the former depends on the latter
Without this commit spice-client-gtk-3.0.vapi may be built before spice-client-glib-2.0.vapi if build_jobs > 1. This causes the build to fail because the former depends on the latter